It's win win, the dealer gets an opportunity to make a buck selling product the bankster doesn't own. Trade-ins have carrying cost, interest, a consignment doesn't.
Most dealers don't keep trade-ins, unless it's pristine or a marque they sell, they shop them to appropriate dealers or it's the auction, just like auto dealers.

Every normal man must be tempted, at times, to spit on his hands, hoist the black flag, and begin slitting throats. H. L. Mencken