The fee to fund the upkeep would still be nothing compared to Ebay and such, because the system could remain highly automated and trustless on a peer to peer level with the DDE.
I think comparing it to eBay isn't the right move for now. Down the line, sure, but right now don't target eBay users. Target people using Bitcoin to buy things, because those are the people who would be willing to put in the extra 15 minutes needed to use this compared to what they're currently doing. The eBay audience would have to put in a lot more effort to make the switch to using this.
The 'BitBay Lite' can be the exact same client software, but with a different skin, so it's super easy to make from what currently exists. If you're looking for something to add, add on something that would make it easier for sellers to use, like a way to organize orders, shipping addresses, and the order status. We already have a reason for buyers to want to use it, so now we need to make sellers really WANT to use it.