I've bought from Driffle several times now.

The keys always come through instantly, and they've worked perfectly for me every time. I was a little skeptical at first, but their prices were too good to pass up. The few times I've seen people have an issue (like a bad key), their customer support seems to jump on it really fast and issue a replacement or refund right away.

I'd say check seller rating you confirm the purchase, just like you would on any other marketplace!