Kinguin

Yes, I think - from what i've heard - that Kinguin is actually safer than G2A. Don't hold me for it tho.

The site is trustworthy. The sellers aren't. xD At least they're not always trustworthy. Some are trustworthy, some aren't, and you can always ask for a refund. :/

Thanks for the input

It is exactly like G2A in its workings, but it is probably the least shady among the similar sites. You are still at a risk, but you have probably the smallest chance to get a fake key, a used key, or a key that will be revoked within months at Kinguin.
Or you could just wait for the same game to go on a sale, since the grey market's prices are always higher than the ones at legit stores.

Kinguin is not an authorized seller.

It's like G2A. All the keys they have are from the grey market. You don't know if it's stolen or not, if they are going to be revoked or not

Yes, I sold a game and bought a CSGO Skin With no issue