No allowing used games will make me buy less games, since i can not sell the game anymore if i don't like it. So games i'm unsure about will not be bought (at least not until they are dirt cheap). Also they'd have to lower the pricepoint because a resellable game is worth more to me, since i can get money for it. If they don't i'm going to ride out my current gen systems longer and become a verry late adopter to get the games cheaper.
Not being able to resell it would make it equal to a digital copy for me and i have jet to buy a digital game at full price.
So while not a dealbreaker it'll still be a loss for them (in my case at least).