Yes CDKeys.com is a trusted site which offers cheap CD Keys on a lot of games. I have bought plenty. If any key doesn't work, they will not only change the key but will refund for you.
HOWEVER, make sure the key you're buying is NOT region restricted, and works on your region. For example. COD Black Ops 3 cannot be activated in Middle East. There may be some games which won't also activate. (I bought the BO3 key seeing it as Global Key, and didn't read the description. It didn't work, for the reason mentioned above. CDKeys staff were more than helpful in refunding my money - took some sweet time, but refunded nonetheless.)
Hope it helps.