Personally I’ve never used G2A before, and probably won’t. There is just way too much shady things that the site has done in the past.
But it does raise the question, how else are you suppose to sell old unwanted game keys. And is it really wrong to sell unused keys? Ultimately selling keys like that is very similar to selling hard copy of games. In principle there should be nothing wrong in doing that.
However, this is the internet, the wild west where the law is 53 shades of grey. Where people will buy stolen credit card details and buy keys on bulk and sell them to people for a profit. This is the issue that G2A has. In principle there is nothing wrong with the site, but because people abuse its services and effectively steal keys from developers by exploiting loopholes, that makes the site a blight in the gaming world.
Steamgifts is cool do to, but it does not benefit people in that they do not get any money from the transaction. Should they get and compensation is another question.