Guy Kawasaki blogged about the top ten barriers to user adoption... The number one barrier? User registration. Hopefully this isn't too much of an issue. Users will most likely need to register with our service in order to use it, but it's going to be simple - an e-mail address and password. It will be part of the listing creation process, and hopefully take only a few seconds.
I don't really see simple registration as a barrier... In fact, it would help greatly to weed out the users that aren't really interested in the system, or those who like to spam. If someone really wants to use the service, taking 3-5 seconds to enter an e-mail address and password really isn't that big of a deal IMO. If users can't do that, then perhaps the service is never meant to be.