I would not mind paying for a copy of an e-book or music recording on the Internet if I could do so anonymously, and it were ethical in other ways (no DRM or EULA). But that option almost never exists.
It is possible to sell online media with payment in cash by mail. Have the payment return a coupon good for some amount of purchasing.
But no need to work so hard at it. Amazon already sells pre-paid cards that you can use to download music and ebooks. You can pay in cash at Safeway.
Postscript: Stallman is against Amazon. And maybe he’s right.