Well, if you don't have full contact details, you are in a bit of a bind.
I assume that this person has chosen not to accept emails from people on this site? If they haven't make that selection you can send an email to the address that they registered with, even if you don't know it.
Does this person belong to other forums? You can use google to search for their username across the internet and you may find out more about them.
The most that I can suggest is that you send a moderator the details and explain what has gone on and we can look at it. However, and I stress this, site policy is that sales/trades are the responsibility of the members involved and getting involved in any way is something that we would be careful about.