sounds interesting, but i do doubt that a decentral network can be used for online auctioning. there are a number of problems i see with it:

how would you rate a user? or how else will you determine who you can trust? i wouldn't like to ship a product to somebody i have nothing but an ip address from. nor would i send him money. you cannot create feedback profiles on a decentral system 'cause there is no central point to save the data.
if you don't get your money there is hardly anything you can do, for you do not have any kind of legal contract.

of course you could sign your orders by some kind of digital key (although digital signatures are not considered legal signatures, at least not in europe), but that would require each user to be registered at a central system like verisign. again, a decentral system is not suited for key authentification 'cause you can never be sure that you have any specific part of data in your horizon.

auctioning usually does require that the user specifies his full name and his full address; hardly any gnutella user would make this information available for the public. you need a system that can be trusted, and a more-or-less anonymous system like gnutella is not suited for that purpose. yes, i know, there is too less anonymity where file exchange is concerned; RIAA can call your isp and ask them to cancel your contract. but there is too less anonymity where legal safety is concerned, that is why not a single gnutella user has been held responsible for copyright infringement before a court so far.

perhaps it would really be possible to do online auctions on gnutella, by eliminating all of these concernes some way or another, but i hardly think that the use would be worth the effort. auctions are an issue which should be handelled on central networks.