Piracy

The problem is not wether the majority of sellers aren't selling keys like that. It's the possibility that some sellers ARE. These reselling sites are massive, and even if it's 10% that are using stolen credit cards - that can put a huge dent in the sellers wallet.

It can even hurt the one who purchasing thinking it was legit if the seller decides to revoke the keys - which they can do at their discretion.

Yet these sites take no responsibility, don't really police or vett the users selling on their site beyond doing just enough to say they are in court, and even happen to offer a subscription model for "buyers protection", essentially allowing them to profit the most off of these sites. I know G2A specifically has been caught making it super hard to cancel these subscriptions as well - it's just super fucking shifty and slimy.