Besides Ebay being the overlord of their rankings, Ebay also has the US government's laws®ulations. If an Ebay purchase were not a legal contract, it would fall apart. In addition to the myriads of consumer protections on top of Ebay dispute resolution.
I would say the way out is through social networking. With Anon+ or whatever its going to be called, you can not only have an individual identity, but many group identities.
Social networking is a good idea. Howabout this: what we need is a general purpose way to establish relationships between people.
Suppose there is a particular group I am interested in participating with. I create a new identity (public/private key pair) for that group. I then begin interacting with those people. I meet some of them and we become friends. I then wish to establish the following sort of relationships with someone:
* Friend = 0.9 (I like them)
* Trust = 0.9 (I trust them)
* Belief = 0.5 (they're a great person and all, but they believe a lot of wrong stuff)
* Sexual attraction = 0.1 (ugly)
These are just examples, but the idea is that there are many ways you might relate to someone else. Often these relationships can be specified with a number representing the degree or intensity of the relationship, and often they are asymmetric (they are my friend with 0.9, but maybe I am their friend with 0.2). Perhaps trust is just one of the ways people can be related. And thus maybe what we really need is not decentralized eBay, but decentralized eBay+Facebook+OkCupid+etc.
(FWIW, the stuff I have described here could be implemented on Proofnet by just broadcasting whatever relationship you have with someone else.)