System Mechanics: Market Value Fluctuation:

Every economic system fluctuates the value of items based on supply and demand.  The higher the demand is compared to the supply, the more valued an item is to that area.  The higher the supply is to the demand, the less valued that it becomes due to over-saturation.  The more there is a need for something, such a staple foods, the higher the demand will be in an area.  Certain items will have higher value to individuals or specialized groups, such as fine art and other rare items, but without the knowledge of its value via Knowledge: Appraisal of its type, it is doubtful that the lay person will be able to recognize its worth and instead buy and sell it at the price that a non-specialized item of that type.  A sculpture that is unusual might be more valuable to a shopkeeper than ones they've seen a lot of, but a buyer might recognize it as one of the totems needed to unlock a hidden temple, or one of the lost works by a famous artist.  All in all, a player would do best to acquire items at low cost and sell them where they will get the most profit.  Knowing the market value of an item may either be acquired for an area by asking around, checking different shops to compare their prices, or if a character has Knowledge: Market Forces II or higher.

Demand / Supply = Market Fluctuation Percentage.

Item Value * Market Fluctuation Percentage = Market Value.

((Item Value / Market Value ) * 100) - 100 = Profit Percentage (Anything less than 0 Profit is a Loss Percentage).

Other mitigating factors such as Renown, Favor, General Charisma, Haggling, and other modifiers may help a character to get a better price than what the individual is selling an item for, but conversely, those same factors may affect said character's willingness to pay the listed price, as well as other factors such as the individual's greed, charity, need of funds for a particular purpose, and other things.  Overall, however, any player should be able to tell if a shopkeeper is overcharging or trying to cut them a bad deal on purchasing an item from the player.  Of note, however, is that most shopkeepers will need at least 15% profit to make purchasing an item worth their effort, so players should expect to receive less than the market value when selling an item, unless the individual purchasing it has some idea of its value.