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.
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.
I should specify that in the BitBay Lite, in order to initiate the contract, the buyer would have to pay immediately. Otherwise, they would be able to initiate and void contracts for low costs that would cost the seller large amounts of money.