Well, this was/is one of the key variables that make consoles easier than PCs, if you take that away, its one less reason to buy a console - mind you as long as GT6 is still coming out on the PS4 only, that's a PS4 sale right there, may be the only reason I buy a PS4, but it'd work.
If its a downloadable game, I find it hard to see how we could sell it back, it obviously will never be refundable, transferable to another console, maybe that's a possibility, it'd have to be really swishy though, like for example, Gamestop have a PS4 and when you go to 'trade in' your game, they'll accept the downloaded game (code) and give you in store credit. But if you're going to want to download more games anyway, in store credit becomes less meaningful, because what are you going to spend it on? accessories? Maybe PSN cards, but that's about all the stores will become good for.
And this is assuming that you can even give your games away at all, AND that the store will be in on it after that. Not looking promising. But hopefully, the prices of games once they become download only, will drop dramatically, to the point where you're only spending $30-$40 on a brand new game, and which point getting back $15-$20 isn't really worth it, this wouldn't be a too bad situation, sure you don't get to recycle your investment, but the initial investment being that much smaller (in AU half price), that you don'' really care as much.