Besides the difficulties for players, there’s also the fact that key reselling is an excellent method for credit card pickpockets to launder money quickly.
Someone’s credit card is swiped in the streets, and in the time before it’s reported stolen, the thief buys many copies of expensive games, to be resold on a place like this. The key resellers themselves claim they’re purchased directly, but…I personally have almost no faith in them or their claims.