This is why, as a seller, I never send anything 
without delivery confirmation/tracking.  I feel 
it is a seller's responsibility, and in his/her 
own best interests, to be able to provide 
evidence when something was mailed.
Write to paypal.  If the seller can not provide 
proof that he/she actually sent the cel, you 
should get your money back- I did when the same 
thing happened to me.  Good luck!