Piracy

Indie game developers have been getting hit with chargebacks for years. To be clear, not every key on the resellers' sites are illegitimate. There are lots of legitimate reasons to want to resell a key, for example a key for a game you're not interested in that's received as part of a Humble Bundle or something. However when someone uploads 1000 keys for a newly launched game, it's highly unlikely that those are legit but the key reseller sites don't ask any questions about where the keys come from. The resellers just want to sell the key and take their cut, and they don't give a shit if it was purchased with a stolen credit card because the original key seller is the one left holding the bag when a chargeback occurs.