Re: cash - A seller can accept cash, but for buyer protection purposes a seller is not allowed to advertise that buyers can mail cash. What's the difference? Cash can be accepted for in-person transactions and it can be accepted for mail transactions as long as the seller does not request or recommend it. For years, my listings have included that "I prefer PayPal, and I accept checks, money orders, currency in US dollars, Canadian dollars, UK sterling, and Euros . . ." I don't suggest or recommend mailing cash, though, so my listing passes muster.
from the eBay payments policy:

"Sellers may accept COD (cash on delivery) or cash for in person transactions."

"Sellers may not solicit buyers to mail cash. Sellers may not ask buyers to send cash through instant cash transfer services (non-bank, point-to-point cash transfers) such as Western Union or Moneygram."

Without getting too lawyer-y, the key is whether the seller "solicits" the mailing of cash. As you might imagine, some buyers prefer to send cash, especially for small amounts or, if they are overseas, for round numbers. It is easy to get American cash overseas, and other than the gap in the exchange rate there are no fees associated with it. As a result, some foreign buyers prefer to send cash. I have also had in-person deliveries on a cash basis in both US and CDN dollars.