Keep in touch with Amazon. Also, start posting negative feedback on your buyer -- he wasn't satisfied, you refunded all his money, and he STILL made 48 negative comments about you. You can also file a criminal complaint against him (depending on his state) for defamation, etc. You can sue him for libel, defamation, interference with business prospects, etc. "thief" & "sociopath" are probably defamatory per se -- you don't have to prove specific damages to recover. However, this will take lots of time & energy for little payoff. You may be able to sue Amazon also - they used a feedback system that created a definite danger of this sort of thing happening instead of the 1-sale-1-feedback system eBay uses. Therefore they should be held liable when this totally forseeable situation developes. (This is really a bargaining ploy to make them remove the feedback.)
Ask amazon if they will allow you to create a new screen name & transfer your feedback, but trnasfer only ONE of the buyer's negative comments & allow you to post a response. Finally, you could just create a sole-proprietorship company & have the company register as a new seller & start building your feedback up again.
This is not legal advice and you are not my client. Double check everything with your own attorney and your state's laws.