I wouldn't think that there is any way for 
someone to have the same email address. Maybe 
he's moved since you bought the cel? =/  Is he 
still selling on ebay then?  Also, when you enter 
his email address into Ebay it will bring up his 
ebay ID. Also something to consider is that you 
can request the users contact information - that 
includes a telephone number. Maybe giving him an 
actual phone call would rattle his nerves enough 
to get his butt in motion on resolving this. Here 
is the page you would go to for requesting the 
information from ebay: 

http://cgi3.ebay.com/aw-cgi/eBayISAPI.dll?
UserInformationRequest   

While it asks for "user ID" email addresses work 
just as good. :)