This is a cool service, and I would certainly use it as a buyer, but to get me as a seller it would need to have an option to get an MLS listing as well. Is there any reason that wouldn't be possible eventually as a feature?

It's just that the majority of potential buyers will be looking for homes through MLS, so you are drastically reducing the pool of potential buyers. Number of interested buyers is going to be a huge factor in selling price, especially if you get a bidding war going.