Well, if you are "big and black" all you need to do is show up at his front door and say "Hi, I want my account back, NOW!", I doubt any pathetic little scammer hiding behind his computer would have the guts to tell you no.
Also, this is why I NEVER buy accounts, RS has the most pathetic low life players of the world when it comes to online games, and I've played a lot of them. It's plain ridiculous how people act and treat each other in this game. Basically, Pixels > EVERYTHING.