I am happy you wrote that. For US domestic sales USPS money orders are pretty secure and were common practice. ebay has encouraged the use of paypal by making it very easy, and sold it with "buyer protection" where the competing methods don't work so well if at all. So, its not mandated I think, but essentially almost all else is excluded. It does have the added benefit for them that there is a good chance that shipments will be made quickly. (good luck if you are a seller who has their inbound funds held up for a period for some reason) ebay sold paypal pretty well, I even had a guy turn down cash with local pickup, he wanted the paypal first, before pick-up... So, I sent it right before a face to face transaction. I did get to save shipping... if he had been the savvy and opportunistic sort, he would have taken the cash and asked if we could cancel the deal officially...

A check is interesting, I have had sellers accept a personal check, but we were both "known" quantities.