He may be a douch, but he kind of covered his butt pretty well with that. You are obviously not a professional. There are a number of things that could have happened while you were putting it in that damaged it, you didn't bother running it up outside before putting it in (the best idea if possible, you know if it works before going through the hassle), were you wearing a grounding strap, you may have shorted the board without even realizing it. Could have also been damaged in shipment.
What form of payment did you use? You may be able to file a claim with paypal if he won't replace it, but I wouldn't hold my breath that you will get anything.
He has a pretty good argument unfortunately and you obviously weren't completely honest with him being he says he won't sell to someone without a site for computer repair. You could try to tell him you ran it up out of the case first, but that will be difficult because if it didn't work before why did you put it in and if it did work before then your obviously screwed and would have busted it putting it in. Sorry, just my .02 man, good luck with it.
