Never had any issues with cdkeys, gamivo and hrk games. I am not sure if they are legit, only greenmangaming has exclusively official keys for every product. But cdkeys has a great service
Free Step Dodge
XxNoriakixX
candynarwhal
CDKeys is probably the least shady of all CD key resellers (aside from GMG, or so I hear); I've bought games from there several times without any issues. It's worth keeping in mind that they're still operating out of a legal grey area, though.