I used we buy any bike for the sale of my BMW GS, I struggled to sell it via trader due to it's high-ish miles. (45k)
I got the valuation which I was happy with, when he turned up he did usual kicking of tyres, making the wrong sounds etc, so I expected a beat down on the price. But the guy offered me £350 more than email price! So I quickly accepted it, they transfered the money and took the bike.
I then received a call a week later, telling me they had over-payed the price quoted due to the collector reading the wrong line on his paper work. Apparently they had docked the guys wages by that amount, because it was his fault.
They had the cheek to ask me if I would pay the £350 back! - I told them it was a process issue your side, did no one check it was the right amount before transferring? As he had to call the office to make the transfer, why was it not checked? Far as I was concerned we shock hands on the deal on the driveway and the deal was done.