I think one of the challenges they'll face is keeping the listing up to date.
If they list a car (a Malibu, say) and it's on a dealer's lot in Roanoke, and then you walk in & buy it, how soon will eBay get updated with that info?
For this system to work, GM's computer network has to extend out to every participating dealer, and those dealers need to have someone whose job it is to enter into the computer what their current inventory is.
I can see, quite plausibly, dealers holding back cars for their local customers, and not entering them into the eBay auction system until it's obvious it's not selling. As a consequence, the eBay auctions will be filled with dealer's dogs.
Chip H.