I hated that system, but to be honest, i saw its positive sides since day one.
I don't even understand why Valve removed it anyway.
Anyway, the problem with G2A is not only with bundle keys being reselled, but also people using CCs trying to 'cash out' some of that money.
I only got something there once and i got scammed, so did my brother, with duplicate keys, Support didn't even want to help and at some point they stopped answering. Thank god we paid via PayPal and we started a dispute to get our money back.
And after i got my money back i got a message from G2A support saying something like "Ah, it looks like you solved it anyway, i will close this ticket.".
And the BEST PART, all negative feedback of a seller are being removed after they 'solve' your issues. This is why you see many sellers with 98%-100%.