A lot of Motability cars go nowhere near an auction, Motability have their own website for selling Motability cars. It is very simple for the dealer you leave the car back to, if they are Registered buyers with Motability to buy the car.
If you are not going to get another car leave the car back to where you got it from but have a discussion with them about buying the car from them. They can get a price for it. If you agree the price with them you can then buy the car from them.