G2A is a shot in the dark, some of them are stolen keys and steam does sales for the same price
and to add on to hardstyle_83 post, I have heard some game developers can revoke your key after it has activated
It is VERY rare that, that happens, I have bought hundreds of games from G2A and never had a key revoked and i've only had ONE issue with an invalid key in which G2A gave me my money back
just bcuz it wasnt revoked it doesnt mean it was a legal purchase, just that no legal action was moved to cancell the transaction (its not the keys that are stolen, its the credit card that were used to pay for the said games.)
On Kinguin you know what seller you are buying from and see his credibility/sells,and even that is a bit shaddy.
ill stick with Steam/origin/Uplay etc, apart from those only humble bundle
edit: all of that ^^ doesnt mean your games are illegal, many of them can come from humble bundle and people just selling the keys from games they dont want anyway