There's currently a site called RollSale (http://rollsale.com) that's doing something similar to this.
There's also AutoTrade.com Trade-In Marketplace where you can get quotes for your trade-in (https://tradein.autotrader.com)
But the problem I see with this business model is that any quote you get from a dealer for a vehicle they haven't personally seen and inspected will not be a reliable bid.
They can always do a bait and switch after you bring the vehicle in. Now that they have you at the dealership, they can wear you down and convince you to drop the price due to "unforseen" issues. Perhaps the vehicle is not in the condition they expected, or there's some mechanical "issues" and they can't honor their original bid.
I don't know of any dealers that will absolutely guarantee a used car bid sight unseen (unless they bid way below market rate), which isn't good for consumers.
Not saying this business model won't work, it's just hard to get guaranteed bids.
Edit: I may have misunderstood the question, but I'm assuming this is a service where you try to sell or "unload" your car to a dealer.