You can request a written receipt from the seller. I've given receipts to a few people I've sold items to. Usually something along the lines of:
Aug 20 2012
Received with thanks from [Buyer's Name] for purchase of used 3XL Jiberish hooded sweatshirt.
Check #209, amount $42.
Sincerely,
[Seller's Name]
Or just use Paypal, and get your receipt of the transaction directly from Paypal.