Grey market key seller? Yes you can get your key banned and maybe get your account in trouble on whichever platform (Steam, Origin, etc) you use.

They sell cheap because they get them illegitimately. Leaked press keys, keys bought with stolen credit cards, keys scammed from developers, datamined keys…

What all of these sources have in common is that the actual developer gets nothing at best, and a chargeback fee at worst. All your money goes to middle men.

Just pirate instead. It’s more responsible.