I sold two cars through Motorway prior to ordering my Model 3 and they were great, although their website does not handle two cars being sold by the same person/PC and gets confused. Luckily a few calls to their service desk sorted that out (their desk queuing needs work though).
In both cases the winning dealer I think had put the funds for my cars in some kind of escrow account and they both used a company called Pro Driver to inspect and collect the car, which was friendly and painless in both cases. They take a few photos (a couple of blemishes which would polish out), then phoned the dealers, said "car looks good...nothing to worry about", dealer then hits a button on the MW website I believe, which releases the funds, I confirm they have hit my bank (can take up to an hour I believe), collection agent then drives away only once the funds have been received and its then an online process to transfer the vehicle via the DVLA website. All very easy and saves a lot of frankly tiresome interaction via "normal" methods.
Dependant on the car, I would also advise getting any scuffs and scratches smart repaired - Chips Away did a great job for frankly peanuts on the tail gate of one car, removing some scratches (dogs ingress/egress marks). This will help regardless of the method of sale used and saves the dealer/buyer having it done.