Decentralized "ebay like" website?

I don't know if anything like that exists.
Maybe there is a Wordpress extension that would provide a bidding system.
but you'd still need to have the trust of the bidders.

One other possibility is a decentralized system that only serves as a way to display an offer,
while the transaction part is dealt with in other ways (meeting IRL, or waiting for paypal/bitcoin payment
and sending the item).
I wonder if such a thing is authorized on Diaspora for example.
Or a diaspora fork specially used to display items to sell would be a possibility.
But that would mean building a community from scratch, which can still be an interesting long-term goal.

It's not quite what you asked, but there is a libre payment tool developed by pehjota: http://pehjota.net/projects/epirts.js/