You should keep it and not mess with the 
complicated ebay thing.  The buyer never finished 
their part of the bargain and has kept you from 
completing the sale.  In the time you have waited 
for them, it is likely you could have sold the 
cel to someone else (and maybe at a higher price 
because the market fluctuates so badly).  It's 
been over 5 months, so I think it is more than 
fair to say that they have "abandoned" their 
purchase.

I'll use an example, say you put something on 
layaway. When you initially put it on layaway, 
you put a percentage of the total price down as 
part of a payment (and as part of the agreement 
to purchase the remaining portion).  However, if 
you do not pick the stuff up within the specified 
time, it goes back on the rack and the store 
keeps your down payment (or deposit).  You have 
no claim to any of the items or any portion of 
the items because you did purchase your stuff in 
the specified time.