Ebay only endorses one Escrow service, Escrow.com. You can read eBay's Escrow Policy here. They only recommend an escrow service for vehicle pruchases, but it is allowed for any purchase. There are escrow fees, so you won't be able to under-cut credit card and paypal users (Fee Schedule $25 minimum).

Ecrow.com does not seem to be adverse to an irreversible payment method like bitcoin. Maybe somebody should ask them if they are willing to make bitcoin an official payment method. That should save on bank fees, but may be a regulatory headache. Edit: I decided to use the site feedback/suggestion form.