I recently sold my M235i through motorway, the agreed price was £2k more than WBAC and the car out to bids from dealers at that agreed price. This price was achieved by a dealer, he came to collect the car and there was no 'bartering' over the price, the car was in showroom condition anyway.I would be very interested in the answer to this myself, as I will be looking at carwow and Motorway values once my car is a full MOT next week. WBAC seems reasonable and then increased a week or so later. A friend used Carwow and highly recommended them to me.
I found the whole experience positive and seamless.