Yeah, I've worked at a used game store as a clerk for two years. What I meant is that its the same concept - basically, businesses makes no money from a used game. If a game is traded twice, thrice and more, game devs lose a lot of money, which is worse when you consider most games only have a ten dollar discount. But I agree that at least the game made profit once, contrarily to G2A.
Beside, I strongly believe G2A is cancer. We already have enough challenging way of promoting games without having a business that fakes legitimity on top of it.