I think a lot of it is risk. You KNOW what $300 is worth, but if you take an item for trade,you may or may not get you money out of the item. Generally, I see higher trade values attached to ads where the seller isn't looking for a specific trade, just says they are open to trades in general. I think of these ads as guys/gals who are buying and selling to try and make a buck, not just get rid of an item.