All I know from experience with a mate who got a price and then when the guy turned up he literally claimed the bike was a wreck (it was a year old cbr6!) was that they earn their commission based on the amount lower than the agreed value they can give you. They weren't the same firm, they were the massive advertised one, but they still did every trick in the book, like a sh*t window salesman the bloke was 'on the phone to his manager to see if he could squeeze a bit more out of him' but my mate literally said to him to f off and then walked inside and shut the door and the bloke knocked and said 'okay fair enough we will pay what we agreed'....... awful.
I'm aware they may try knock me down, but it's cost me nothing to get them to my door. They won't be taking the bike if I don't get what I'm after. It's the superbike factory who are buying it, they need more stock they say.
Update- Superbike factory turned up. Inspected vin and engine numbers, checked electrics, checked paperwork. Bike was as I described, he rang his office, and the money was in my account before he got the ramp out of his van. Did not try to haggle the price at all. I got my asking price and the easiest sale I've had. It may not work for everyone, but if your realistic with your prices, it's well worth using them. No eBay messers ECT to deal with.
