I purchased a COD4 cd key from www.cdkeysdirect.com
two months back, based on a few recommendations on another site. I was emailed the key from an actual person maybe a day or less after PayPal transfer.
My second attempt at logging in, it said CD key in use. I emailed the person above who responded in less than 24 hours on the weekend, saying he tested it, and it works. I was a bit perturbed that he can and does test the cd key (and admitting to playing around), but maybe that is just me. When I reinstalled Vista, I ran into that again, but a restart fixed it which makes me think that both problems were somehow on my end.
PM me if you want his email.
Also, I read on some cd key seller website that they purchase the keys from gamebox sellers who either cannot or do not sell the gamebox anymore, whether it is due to a change in business plan, admin problems or something else. I didn't buy my key with 100% certainty that it will always work; I assumed it wasn't going to work, but the difference in price was worth the risk. Works so far.