On the dragonsfoot forums recently, it was asked whether experience points should be awarded for the "real" value of non-monetary treasure like gems, jewelry, and works of art, or if it should be based on the price the characters get for it when they sell it.
In a very basic game, in which many things outside of the actual adventure are hand-waved away, you could just assume that the book price is the selling price is the XP award, and leave it at that. Personally, I like the world outside the dungeon to have a little more depth, and that includes decisions about whether to accept a quick turnaround on looted items for less than book price, or to hold out for more. That in turn means that the selling price of any given item is likely to be less than the theoretical "book" price, and there arises the dilemma mentioned above.
To me, this is an incredibly easy question to answer. Adventurers earn XP for their success at adventuring, not for their success, or lack thereof, at being merchants. Whether they make a killing on their loot, or get taken for mere coppers on the gold piece, or keep their baubles as trophies, or give them away to beggars or comely barmaids, they did their part as adventurers. They shouldn't have to prove themselves all over again by ferreting out potential buyers and making sales pitches. That's the job of a merchant.
I would go so far (or at least, my inner economist would) as to say that the rules as written in this case have it exactly ass-backward. The book value should, first and foremost, be an XP value. Then that XP value can serve as a guideline for determining a sale price.
Warning: Economics ahead
What we have here is a very nice fantasy world example of the economic principle known as division of labor. Merchants don't go rooting around in dungeons and lairs to salvage merchandise, because they are poorly suited for the task. Not only might they be unsuited physically and temperamentally for it, but adventuring requires a substantial investment in equipment and training, and obviously time and money invested in equipment and training for adventuring can't also be invested in equipment and training for commerce. A good merchant spends a great deal of time and money building his reputation, learning how to recognize the quality of goods, and cultivating connections so he'll know where goods are in greatest demand and who might be interested in unusual items. A good adventurer spends a great deal of time and money learning how to fight and/or cast spells, assess danger, and survive in hostile environments. There's just not a lot of overlap between the two skill sets, and extremely few people have the time, inclination, and talent to learn and excel at both.
Another difference between merchants and adventurers, that makes it profitable for both to do business with each other, is their respective time preference (if I may resort to the parlance of the Austrian school.) Adventurers want coin they can spend, and they usually want it right now. Merchants are willing to hold onto the goods a while in order to seek out the best possible return. Because of time preference, present money is always worth more than future money. When given a choice between $10 now and $10 next week, who wouldn't choose to have the money now? In order to persuade you to take money next week, I'd have to offer you more money. How much would it take to get you to pass on the immediate $10 and wait a week? That depends on your time preference. The higher it is, the more I'd have to offer to get you to wait.
How does this apply to Joe the fighter? Let's say Joe found an exquisite bracelet, and somehow has found out that similar bracelets have sold for about 500 gp. If Joe needs cash now - say, he's gotten himself cursed and badly wants to donate to the temple and have it lifted as soon as possible - he might not want to mess around for a week looking for a buyer to pay him 500 gp. He might be perfectly happy to sell it to a pawnbroker for 250 gp on the spot, because 250 gp now is a lot more valuable to him than 500 gp later. The pawnbroker, having a much lower time preference, is willing to let it sit on his shelf until a buyer makes him a better offer.
This isn't to say that the pawnbroker has no time preference at all, though. If he knows of a likely buyer for the bracelet right off the top of his head, and could resell it tomorrow, he'll be willing to pay more for it than he would if he thought it might take him a month to sell it. Let's say Joe wants to bargain Peter Pawnbroker all the way up to 450 gp. In the first case, Peter makes a tidy 50 gp profit in one day, but in the second case that would equal a 50 gp profit in one month, and Peter isn't willing to tie up that much money for that long if he's only making 50 gp out of it.
However, if Joe isn't particularly concerned about having coin in his pocket right away, he could still take advantage of Peter's reputation, knowledge, and connections, and get a better price for the bracelet than he could if he tried to sell it himself. He could offer the bracelet to Peter on consignment, and Peter would pay him a prearranged portion of the proceeds when it sells, say 85%. Peter doesn't have to invest any of his own money up front, and Joe gets the benefit of having his bracelet on display to all Peter's regular customers. Peter still has an incentive to use his skills and knowledge to get the best price he can for it, because the better he does on Joe's behalf, the bigger his commission, but he can afford to wait if he must, because Joe is paying the bulk of the "cost" of time. It's Joe who's going without the services of several hundred gold coins until the item sells; Peter is only deprived of a relative few.
The upshot of all this is that, unless he's just outrageously lucky, realistically Joe is going to be better off using Peter's services as a middleman than being greedy and trying to go it alone. This holds true whether his time preference is high or low. In fact, the amount Joe gains, if any, from stubbornly insisting on the
"full" price of his jewelry may well be less than what he could earn if
he headed right back to the dungeon instead of wasting his time wheeling
and dealing. The only time it does not hold true is when the DM decides to be unrealistic and let Joe do things that he shouldn't be able to do very well without a merchant's training and connections. (Not that there's anything wrong with that, if that's the way you play. Just don't kid yourself that it's realistic.)
(End of economics)
How can all this work in play? Let's start with the assumption that a merchant knows pretty well what he could get for any given item. It's going to be hard to pull the wool over his eyes without some deliberate and clever deception. Even though it's not kosher with economic theory, a piece's XP value is a decent rough estimate.
Roll 2d4 and multiply by 10; this is the percentage of a piece's XP value that a merchant won't go over. This doubles as an estimate of local demand and the merchant's savviness of the local market. A low roll means he expects great difficulty or a long wait to sell the item, or perhaps taking it to a larger settlement. A decent roll means he has fair confidence that somebody will want it in a reasonable amount of time, and a very high roll means that he has a prospective buyer or two specifically in mind.
Modifiers: +1 for large town, +2 for city, -1 for village, -2 for hamlet or outpost, +1 if located on major trade route, -1 if unusually isolated.
In order to keep things fairly consistent, if the PCs go looking for another opinion, roll 1d4-2 (min. zero), and add or subtract (50% chance for each) the result to the initial roll.
This doesn't mean that a merchant will start the bidding at this amount, only that it's the highest he or she is willing to pay. The opening bid might be 50% of the maximum, or even less, depending on the temperament and honesty of the merchant. It's up to the players to bargain for a higher price if they think their goods warrant it.
For simplicity, you might wish to process batches of very similar items with a single roll; if the party has, e.g., five gold rings to sell, it's not likely there's going to be wildly varying demand for them. If they want to sell a large and varied trove all in one go, just assume a roll of 5 and apply the appropriate modifiers - it would likely average out about the same if you actually went to the trouble of rolling each item individually anyway.
Terms for consignment are left to the PCs and merchant to hash out, but should generally be considerably more favorable to the PCs than selling to the merchant outright. A merchant will probably demand a larger share for big or bulky items that take up more space in his wagon or shop or are otherwise just a pain to have to deal with.
Personally, I would refrain from giving PCs any more than vague clues about the value of an item when they discover it. This should help avoid any resentment for "cheating" them out of the "real" value of the items when they sell them if your players are so unfortunately inclined, and makes for more interesting decisions about what to carry out of the dungeon. It also encourages them to form relationships with merchants and experts, and learn whom to trust, which can make for some interesting role playing opportunities. (These don't have to be long and involved; even a quick exchange can add to the game. Recurring bit characters are fun, and if you ever decide to have a merchant robbed, kidnapped, or killed as an adventure hook, you've got a better chance of getting genuine reactions from the player characters.)
Again, regardless of the price they get for their bags full of shinies, the PCs should receive full XP value. Not only is this great for realism and verisimilitude and all that, it's a great way to keep the XPs coming while curtailing somewhat the amount of cash flowing into the PCs' coffers.