I've bought games four times in the past from key resellers, mostly to circumvent censorship. I think one was from MMOGA and two were from Kinguin, and one from another site that I normally bought PSN credit from. I've personally had no problem in any case, but MMOGA is a fairly respected site and the seller on Kinguin was very highly rated. Of course, neither case is a guarantee that you won't get scammed.
The more you do it, and the more money you try to save doing it, the more likely you are to get scammed. That's a simple fact of life. If you buy a key of a $60 game for $20 on a key reseller site, it stands to reason that something isn't quite right. And yes, G2A is one of the places that you are mostly likely to get scammed. I suspect the ratings there are manipulated.