Well, it is a bidding process. So if buyers believe there is enough added value in this service, they will overbid the 'CL price'. So if a couch would sell for $300 on craigslist, presumably it could sell for that plus whatever the value of having the item delivered, let's say $400.
I think the bigger issue with this service, is that the cost of delivery of a large piece of furniture is relatively constant regardless of the value. So if I want to sell a $1000+ dollar couch, I have no incentive to use this service, it would make much more sense to sell it on craigslist and offer to cover the costs of a professional moving service instead ($500 vs $100-200). It seems to me that this service is most valuable for lower priced items. So if I can sell my crappy $100 couch hassle free, but only get $50 back, I would be pretty happy with that. However I doubt that would be profitable for the company.