A handwritten receipt with a date and purchase 
price is just as good as the one you get at a 
store (or so I have been told). If possible, you 
should contact the people you bought the cels 
from and ask for a receipt.  And if any of them 
were purchased on auction, get both a receipt and 
make a copy of the auction page. Just make sure 
you don't go into battle "unarmed".