‘Barn find’ is ebay UK shorthand for ‘got it in a house clearance’ or ‘have to get rid of it as it is a rusting heap’.
In reality they have been buried at the back of a garage or damp shed and some (if not all) of them will have been left there because they had a problem or were worn out and the original owner was ‘gong to get round to it…..’ Eventually.
Obviously leaving them there for 20, 30, 40 years or more will have improved them immensely. You can pay daft money for the rusting trash certain that, as long as you tell people how wonderful they are, they will be a valuable classic, ‘better than money in the bank’. Or not 😉