G2A? I can tell you. From what I know and what I suppose:
* part of them are Steam codes, which have been offered as GOG codes by
mistake (99,9% of G2A are Steam keys and some sellers are not aware anything else exists) - please, note that there are a few offers containing games from the last Humble Bundles (but all that bundles were Steam keys only!)
* part of them are keys for games, which offers
digital copy for box/Steam owners (as from
that list of games) - some people are not interested in digital copy and I suppose they're selling their code, keeping just a box/Steam version
* part of them are keys
bought during sale and offered when the sale is over - in such case you'll pay more than just waiting for the next GOG promo, because you need to cover high fees and seller's profit
So even considering that none of that offers is just reselling codes bought with
stolen credit card, it still a better option to just wait a while for GOG sale. That's why wishlist is a great idea ;)
Marketplace as G2A makes sense for Steam, where you can buy codes from
different sources with huge discounts, including
bundles and giveaways, which duplicates some items in your library. In case of GOG, which is the only source of GOG codes, it makes no sense to look elsewhere.