You can buy keys on (appearently) unlimited stock from the system itself (And its afaik the only way to obtain them, so also not like you could get 1 droped and then sell it underpriced because you have no need).
But I see soooo often people placing keys on the communitymarket, some times even the cheapest offer starting by + 0.50€ more then the price I had to pay them directly to obtain from system.
Is there any reason that 1 should buy them from communitymarket for higher price?
Or are this just offers hoping for some one stupid who doesn't know?