Worst thing I didn't figure out how to buy: When we were about to leave Germany in 1972, the main clock dealer through whom I had bought most of my TR items offered a complete SS general's uniform; breeches, tunic, overcoat, visor cap. Price was $1100. I was sixteen and didn't have that sort of money and my dad was working a house buy at our next duty station and didn't have a grand laying around either. Could be a significant part of my retirement portfolio now if only I had figured out a way.
Worst thing I did to a perfectly nice artefact: Bought a clean M16 stahlheim, good paint, good liner. I had my dad sandblast it to bare metal and then I krylon spray painted it black and put some cheesy Leibstandarte decals on it.
Fortunately by the time I switched to soviet militaria from german, I learned not to monkey with anything. (Although I remember passing up a pair of ORB screwbacks with documentation for $500 shortly after I started, thinking that was a bit pricey.)