Well that would only be a matter of if anyone called to say its been sold
But
If i agreed £20 with some and found out later it's worth double, then that would be my tough luck for not researching the item I had. As I would have tied myself into a verbal agreement and that's the problem these day people not sticking to their word.
I had an air rifle and told a chap I know 30 so be said yeah I'll have it pick it up in a few days and I hadn't taken all the ads down and in the mean time I had another 2 / 3 people ask offering 40 - 60 but I told them it gone for the simple reason it's not right