I think this is a very good idea, and it's truely car dealing for the 21st century. Although how annoying will it be if you buy a car for £12458.57 and then another one goes for £11957.28 the next morning!
A substantial amount of a cars price is overhead for the dealership; not just comission to the dealer (although this is significant) but also their base salary, the ancilliary staff and the cost of keeping the potted plants. By cutting that out they can sell the car more cheaply.
I also think this will be popular with some customers. Everyone will be able to see what other people are actually paying for their cars, so they will know how much they should pay to be getting a good deal. If someone doesn't like haggling, and isn't confident dealing with slick salesmen, this could be very attractive.
They'll need to make dealerships, or viewing centres, available for people to test drive cars. Plus they will need to work out how people can select options on the car, perhaps they could buy a standard trim level then buy options after the acution. But either way I hope this takes off.
