Funnily enough, I'm just on the train to go and look at a SOR car... I work in the 'modern classics' market so it's not unusual for collectors etc. to want to move a car through a dealer rather than having to deal with Joe-P.
I charge £1000 or 10% (+VAT), whichever is higher. For that they get the car detailed, advertised and deal closed etc. I have my own T&C's which the client has to sign before I take the car - key point on there is that any expenditure required to make the car 'retail worthy' is carried by the client, not me. There are other conditions which are there to protect both me and the client, but fundamentally I would not retail the car unless it was as per one of my own.