Again, I refer to my 'selling-your-2nd-hand-car' analogy.
Selling it to a friend (who can't afford new cars/games) in order to put down a deposit on a new car (read new AAA title game) promotes ongoing commerce. Selling it to a dealership (read BT-Games) will get you less money for it - and in turn they'll put a markup on it and continue making money off the sale even if the supplier (car manufacturer/game publisher) only sells it once.
If they want to make money, lower the prices of digital releases significantly (as the overheads are next to nothing) and that way it's sold for less, but can't be resold.