ebay has an 'adults only' section and paypal can be used, but the rules there are pretty strict. Also quite specific...porn videos (produced by a "legit studio") are allowed, but "digitally delivered adult goods" aren't permitted. I'd take that to mean selling a hard copy of a porn video is allowed, but selling the same video as a download/pay-per-view/members area vid online is not.
Not very useful for most of our purposes :/
here's a link to the Ebay adults-only section rules:
http://pages.ebay.com/help/policies/...only.html#what