The thing about farm sales is, particularly if the farmer is retiring, the neighbouring farmers want to give him a good send off. So if there's something there they can use, they'll buy it. They don't care what the retail price is for a new one, they just want to buy something from Old Fred's Farm Sale, help Fred on his way and have something to remember him by. I've seen rickety old wool sack frames go for twice the new price because it was the only half useful thing left to sell!
All the unsuccessful bidders had to pay tenners for bags of bits of screws, old dosing guns that probably don't work, rusted up old G-clamps, caked dry branding fluid... you get the picture.
So if you don't know Old Fred from Adam, make sure you know the retail new prices of things, and only buy things that you'll use and are good value to you.
And every time you see something go for more than the new price, don't think "farmers don't know the value of things
", instead, think how lucky you are to live somewhere where the local folk
really know the value of things, and what things are of value.