How does this work, .., does the player send you the login credentials?
Basically yes. A lot of the Chinese sites offer powerleveling and other services like that, where you would be supplying your credentials. The reason why I'm not just selling gold is because all of my 85s are currently on one realm, and unless you happen to be on the same realm, I'm pretty limited in what I can offer.
It's much easier to just farm up gold or honor or whatever on your 85 character, or just powerlevel one of your low level characters. As for league of legends, yeah, just need your credentials. In LoL I've played with most of the champions, so no matter who you have, I'll be able to play and win.