I get where you are coming from as I’ve been involved in group buys before and you get an ‘x’ amount off.
with there being so much choice it would be hard and would have to be done on a manufacture deal such as Volvo, VW etc as opposed to a specific model.
ie. VW (wherever the location may be) get 5 people and get £100 off your orders.
It sounds good but you have to factor in distance, time, mobile needs etc so IMO will be difficult to do.
good idea though.