Honestly the best way to avoid it is Delivery 
Confirmation its cheap only $0.45 or cheaper 
depending how you send the package.  And if 
someone sends a complaint just send paypal the 
tracking number.  D.C. doesnt really track the 
package it just verifies that the package got to 
its destination.  I have had buyers pull this on 
me as well but I have never had a problem with 
D.C.  Now this only works if you live in the US 
and are shipping within the US.  To make up for 
the cost add $0.50 to your shipping amount.  If 
you are international charge the people for EMS 
or if you are sending international from the US.  
Make them pay for the express its a little pricy 
but they get the cel/item fast and no complaints 
and it has a tracking number.  If you describe 
your situation is detail to your buyers on your 
policies on shipping they tend to be more 
forgiving.
Anywho I hope that helps
Ordinary