It has been suggested in the past that this be done in the buy/sell area as it is a form of buy/sell.
Both parties in a trade agree that a particular item has a certain value. They will only trade for something of equal or similar value. Once you declare a value for an item and want something for it to match that value, you are selling it. In fact, in a trade, instead of buying something or selling something, you are actually doing both at the same time.
MP