Over at Gothridge Manor there’s an excellent post on valuing gems. In a comment, I replied with my own (admittedly simplistic) version of gem valuing for S&W. If you’re interested:
Whenever a player tries to value an art or treasure piece, I secretly roll a d10. For every number BELOW 5, they undervalue it by 10%. For every number ABOVE 6, they overvalue the object by 10% (this can make for some regretful discards when they players are trying to decide what to load onto their mule). A 5 or 6 means they get the real value.
Tim adds the character’s INT bonuses and many social factors (style, etc.) into the equation, but I simply am not a fast enough thinker on my DM-feet to do this (plus, we play S&W, so there’s no INT bonuses). If a player has 15+ INT, I will adjust the d10 roll one closer to the 5-6 sweet spot (I do not tell my players this rule).
When the players go to sell in town, they will receive the price they ask if they are undervaluing the object (I don’t allow haggling in my game unless the players are buying or selling something story-related or epic in value, such as a warship or keep). If they overvalue the object, the shopkeeper refuses and will only give them the correct value of the object.
If players suspect that the object is undervalued, they can pay an appraiser 10-15 gp to analyze an object in any town with at least 1,000 people in it.