True. Some guys simply say “if you want to trade it has to be in my favor.” Basically the same thing.It's really not that difficult of a concept to understand...
Seller has $1000 MSRP item listed for $500 cash as the item is in pre-owned condition. Buyer has a $500 MSRP item that is also used, but they feel that the items value is $500 as that was the price they originally paid (typical OSA buyer logic). This is why trade value is higher. I'm not trading a $1000 MSRP item for your $500 MSRP item (usually some variety of Glock), just because my cash asking price is $500.
Most sellers, at least the successful ones, will ask less than market value in order to facilitate a quick cash sale. If turning your asset into cash is the objective, why would you be interested in accepting a trade that is valued less than the item that you're attempting to sell?