I have the same thing with an account lol. Have you tried adding the game in-name to see if you can find a previous name? Because other than that I'm not sure how, if someone could find out that would be sweet.
You have to be pretty lucky if the in-game name is either the account name or if the previous name was it. After a few name changes the account is pretty safe from being found out it's real name.
You could always try to SE the username. Add the person in-game. Make friends with him. Ask him you will buy his account, and assuming that he will sell it to you, ask him what his real username is because "I like accounts with original login names" And if he says it is not very original, tell him "I will be the judge of that"