My best garage sale finds were all made about 20 years ago.
At the time, M42 lenses were so unfashionable no one wanted them and most camera shops in the UK refused to deal with them. I bought six Carl Zeiss lenses for £19 each. These were from different sources, £19 sounds better that £20 I guess and seemed to be the standard price.
Just recently I managed to sell these on eBay for over £100 each. Suddenly everyone wants a Carl Zeiss Sonnar or Flektagon and M42 are the most popular, as they are the easiest to get a mount for. One Flektagon went for £180.
Bizarre thing is, for every lens I have that is badged as 'Carl Zeiss' - I have an identical one badged 'Pentacon'. Whilst we all know they are essentially the same thing - on eBay anything Pentacon is almost worthless...