|I'll give you 10 gp, and not a copper more
Currently, when I'm writing a hex crawl, I'll include treasure hordes with notations like "large ruby worth 5,000 gp". What if, instead, I merely wrote "large ruby" and let the value be determined by the customer?
The basic idea: Come up with a matrix. The columns represent different classes of customers, the rows different categories of treasure. The data would be a random amount of money that the customer would be willing to pay for the treasure. The GM would roll this to determine the starting bid, and then roll a second dice to determine how high the customer will go. Adventurer and customer (GM) could then work out a final price for the item by haggling.
Classes of Customer
Peasants: These are your average working stiffs - laborers in towns and cities, people who carry things and serve others. They didn't make much money in the real world - some would figure it at the equivalent of 1 or 2 copper pieces a day - but in the fantasy world, the standard is 1 silver piece per day. Either way, they have expenses, so they can't afford to spend much on luxuries like treasure. There is a 90% chance they'll offer goods and/or services instead of coins.
Artisans & Traders: These skilled laborers make a bit more, maybe five times as much as the peasants. This gives them a bit more money for luxuries. Still, if adventurers are going to these guys to sell their treasure, they're probably a bit hard up. There is a 75% chance they'll offer goods and/or services instead of coins.
Merchants: The merchants have plenty of money, though their assets probably aren't liquid (meaning they have lots of stuff - goods, wagons, camels, ships - but not lots of money). Still, they aren't hurting, and they can drop a few coins on the good things in life. There is a 50% chance they'll offer goods and/or services instead of coins.
Merchant Princes: These are the big-time merchants, the fellows with royal and noble connections that allow them to own fleets and caravans and manors, etc. They're going to be a bit more liquid than the common merchants. There is a 35% chance they'll offer goods and/or services instead of coins.
Aristocracy: The lower end of the titled fellows - the knights and baronets and such. Like the merchants, their wealth is mostly tied up in things - land, animals, armor, weapons - so they're like uber-barterers. They have a few coins stashed away, but they're probably more apt to trade things like armor, horses or favors. There is a 65% chance they'll offer goods and/or services instead of coins.
Nobility: The nobility includes barons, counts, and the like. Lots of land, but, as with the merchant princes, more liquid than the aristocracy. There is a 25% chance they'll offer goods and/or services instead of coins.
Lesser Royalty: A step up from the nobility - the dukes and bishops. There is a 20% chance they'll offer goods and/or services instead of coins.
Greater Royalty: Kings, queens, princes and princesses, and archbishops as well. There is a 12% chance they'll offer goods and/or services instead of coins.
Imperials: Not Chryslers, but actual imperials - emperors, empresses, kings-of-kings, popes, etc. There is a 6% chance they'll offer goods and/or services instead of coins.
Categories of Treasure
These are the same categories you will find in Blood & Treasure, and adapting them to your favorite game shouldn't be too taxing on the grey matter.
Fancy Stones - agates, hematite - the stuff you find in shopping malls and tourist traps
Gems - better than stones, not as good as jewels
Jewels - rubies, emeralds, sapphires, diamonds
Common Arts & Trade Goods - armor, weapons, things made out of non-precious metals, common animal skins, rugs, many tapestries, common sorts of books. Assume the price is per ounce where applicable.
Fine Arts & Exotic Goods - lacquered wood, rare spices, items made from precious metals, bejeweled items, the skins of exotic animals, rare books, especially fine paintings and tapestries. Assume the price is per ounce where applicable.
Minor Magic Items - potions, scrolls, magical oddities
Major Magic Items - that stuff you really want to put on your character's equipment list
The table above is a simple matrix. Find the category of treasure and the category of customer, and you get their opening bid. Roll a d6 to find out how high they'll actually go:
1-3: No more than 25% higher, and they might have some conditions
4-5: No more than 50% higher
6: No more than 100% higher
Also, remember that there is a percentage chance that the customer offers to pay with goods and/or services rather than actual money. The value of services rendered is up to you, but most games give some sort of guidance. Favors are tricky - they may not be honored at a later date - but they could come in handy.
Obviously, some interpretation is involved here for the GM in terms of treasure category and customer category, and feel free to apply other factors. In a country where gold or silver is common, objects made from gold and silver might be considered common arts rather than fine arts. Likewise, spices, furs and pelts might be common one place and exotic in another.
The impetus for this table was a painting I posted a few weeks ago when I asked the question "Are Treasure Hordes Too Small?". The idea here is that you can now provide a fairly large horde without having to predetermine what everything is worth. This system also gives adventurers a reason to make contact with nobles and such, which in turn can lead to further adventures.