[kalleth]

Forgive me for being potentially obvious, but can't you trace every single bitcoin transaction ever?

So you can go from:

Domain seller -> "Anonymous persons bitcoin address" -> Bitcoin address of the person who sold him those BTC.

You then find that person (as i'm pretty sure they're not so focused on anonymity) and wrench-attack a description of this guy and location + time of the meet from him (and also the e-mail he sent to arrange it -- tone, etc).

CCTV camera footage of the meet/people in that area at the time, etc. Home and dry.

[mr_luc]

Not if he uses a mixer to pay you the bitcoins, no.

Blockchain.info has a fantastic almost-free one.