You know what, that's happened to me the other 
way around.  I've sold things and (since getting 
money is quicker than getting what you bought) 
couldn't leave feedback because my bidder was 
unregistered.

Maybe this is the same thing, for some reason 
your seller was in trouble from an earlier 
auction and got suspended by eBay.  ?  I dunno, 
just speculation.  

Okay, sorry, I didn't answer your question!  
^_^;;  I have no idea if you can reactivate an 
account.