okay...so lets run this down piece by piece to make more sense of it, Marcus is your account, your friend sold your account, the person who bought your account took his money back after he got your account number.
Mkay then, first, i do feel sorry for you, don't get me wrong, anyone who looses an account in any way has my sympathy. Now onto the stuff you'll prolly hate me for.
First. you said your friend sold your account....why? you haven't explained that part very well, did you tell him to? did he do it behidn your back then tell you? I think this is the part where you loose most people.
Secondly, bringing up proof on an MMO is borderline next to impossible, for a few big reasons, a. you can't see the person IRL so you don't know if they just wanted to reinvent themselves or cause a little drama or w/e (please realize i'm not saying this is what your doing just making a point) next, even if you try and ask said person something you know only the two of you know, they can easily feign stupidity and pretend to not know the answer. because if you respond then your busted.
Lastly, this is a classic case of he said she said, its your word against the other persons, this person could easily say you gave it to them, and you suddenly changed your mind and you wanted it back but said person didn't want to give it back to you. so then they'd say your only in here to start a slander campaign.
So i'm sorry to say, the odds are stacked heavily against you, which sucks, because i know i'd do the same thing your doing if my account got jacked, but this is where the don't share your account with anyone for any reason comes into play, even if they are your friend, don't give them the info, friends aren't always going to be friends. (some do mind you) But its always better to play it safe, my brother plays this game, and he doesn't have access to my account, nor would i let him have access, unless i was in the same room and i logged myself in, since i am a 60 whm and a 70 SMN he often uses my char to PL his wife and sometimes double boxes to PL himself, so you see, he uses my account but he doesn't know the password to access it.
I hope all works out for you, and please don't think this is me trying to burn or flame you, i'm trying to help you understand your predicament and also what you need to tell us to help us understand your situation, details are key in these sort of things.