I just lost a 625 receiver to dish network.. I bought it off of ebay over 3 years ago.. I couldn't prove it. It wasn't a big deal.. I only paid $80.00 for it and used it for almost 3 years.. But I could not come up with a receipt because of the age on Ebay..
Moral of the story.. "Keep your receipts"
If you paid with PayPal, THEY will have a receipt of the transaction.