I've lost money on things but I wouldn't class those instances as mistakes as I've gone into buying them (usually new) with my eyes open knowing what will happen as a result of my gas.
99% of my purchases are used items off BC. That's the joy of buying used, you can try lots of different kit and not lose on it.
However just to add to the Wal hall of shame, I bought a used Mk1 for £575 from Carlsboro in Nottingham in 1993 and sold it for I think £650ish a year later. At the time I was quite chuffed with the deal. The next used Wal I bought cost me £3000 🙄