I have had numerous auctions where people have accepted cash by mail. It works out very nicely, and it is not against the rules. Auctioning off CASH or debit or Visa cards is against the rules.
I really suggest you reconsider your PayPal only idea. The more shipping options you have, the more people will bid on your items. I for one am very interested in this game, and I will drive the bidding pretty high if I know that I can pay by cash mail. Either way, you will still get all of your money.