wow seriously? junk trading derives from a combination of natural buyouts and manipulation buyouts. when an item is bought out (examples: partyhats, a newly announced clan manipulated item), because tomorrow it will be worth more than it is today (and by that reasoning the third day more than the second), people value the item higher than its current market value (sometimes vice versa, esp. with new useful items, like staff of light shortly after release). when an item that was bought out reaches a critical point and reverts to crashing, because tomorrow it will be worth less than it is today, people value it less than its current market value (sometimes vice versa, like first couple days after climbing boots update). so now a player who wants one of the bought out items (which yield profit) is willing to take some of the crashing items (junk, which yields loss) in the trade for cash (constant value). the seller who wants to get rid of his crashing items (junk, which yields loss) must add bought out items (which yield profit) to even the sale for cash (constant value). tl;dr - junk is overpriced and "street items" are underpriced; people sell them together for cash to make the trade fair.