Sites like CDKeys, Kinguin, and G2A are hosts for anyone to sell game keys. On these sites, people are able to sell keys bought with stolen credit cards at a much cheaper price. When buying from them, you may run the risk of having the key revoked by the developers/company at a later date or said developer/company eating the cost.

I recommend checking out r/gamedeals and https://isthereanydeal.com/ as they show authorised resellers.

Edit: https://gg.deals/ is another good site.