Send him one last email and then after that I guess all you can do is downvote him in Feedback and leave a comment explaning that he didn't even make the trade. I don't know if something extra happens if a seller fails completely on a transaction so maybe try and find some sort of mod or site manager and ask what the policy is on that and what to do about it.
Mayhaps he was silly enough to go on vacation while the auction automatically relisted his bid. If that's the case he may contact you and apologize or something. When that happens try and get some sort of severance package from him for not being able to sell you whatever item you wanted in the first place.
