Well what you should do is tell him the truth you 
sold the cel. Anyway he never should have sent 
money through the mail because the post office 
doesn't take responsibility for any cash sent 
through the mail.  Well what he should have done 
is sent a money order or cashiers check instead 
of cash. Sorry to say even though it was 
registered anything could have happened to it. 
Thats why the post office says never send cash.
Anna there really is nothing you can do you sold 
the cel already. 
Hope this helps 
concerned