G2A has it's sellers, it's like an ebay for video games, sellers have ratings and feedback count, that's how it works.
It's very hard to find a scam but if you do it's them giving you a different item.
It's safe to use and I bought Black ops 3, fran bow and RWBY from there.
Dont waste time with g2a guard either, just make sure the seller has a high rating.