An addon that keep track on the value of the item after visits at the vendor. An example of this addon is made for WoW (World of Warcraft), called "SellValue" (
http://idarlund.stud.hive.no/SellValue.jpg).
This is how it worked in practice:
After you had visited a vendor. The items you currently where in possession of and that where listed in the "Sell" sections where stored in a database. If you later on moved your mouse over the item, a window would pop up with the description of the item including the price that the vendor would give for it (if multiple vendors had different prices, the highest price where stored).
This is usefull for everyone that wish to sell their items at the market. Would then make it easier to sell it above vendor price, instead of overflowing the market with vendortrash.