Since this guy has absolutely no interest in making this right I would like to take over ownership of his account/change the username. The account age/post count has value and I deserve something to cover my losses
This is an interesting proposal.
Tough call, in my opinion -- you are responsible for engaging in a transaction without escrow (can't knock you, though, as I hate using escrow as a seller), and he's responsible for controlling his account. But again, in my opinion, if he is truthful, he had no malicious intent, and so it's only the hacker you should take issue with despite your desire for compensation.