IMO, based on your side of the story, I'd send 
him one more email, an ultimatium, tell him to 
respond in 24 hours and require payment in 48 
(via paypal only); failure to do so, forces you 
to leave negative feedback. Harsh? Not really, 
sounds like this guy is yanking your chain. If he 
was serious on paying he'd make the effort. 
Accidents can and do happen... but should that be 
your concern? I for one don't spend the time 
figuring out who is lying and telling the truth 
whenever someone cops an excuse. 

And besides, I don't know how anal you are about 
having 0 negative feedbacks, but personally I 
don't care, especially if the deadbeat I leave it 
for clearly earned it by failing to pay within a 
reasonable time (as outlined in your auction 
terms). If you don't have such a thing, perhaps 
you should consider adding one for next time.

Lastly, I'd say if he fails to pay after you've 
graciously given an extension, the hell with him. 
If you feel the need to explain, do a followup 
comment on ebay on whatever negative feedback he 
leaves you. And better yet, make a link to a page 
explaining your side in detail for anyone 
interested in knowing more about the negative 
feedbacks you received.