I don't blame a lot of sellers, I blame the service guys who tell them it's not worth fixing, and they should just by a new machine.
Some of my best scores have been 70 - 80 year old machines that appear to be almost unused, and they have something jammed up into the bobbin case - once removed, they sew perfectly!
I've also gotten a few prestine parlor cabinets and machines. I know these weren't cheap when new, so I like to imagine the wealthy husband being talked into buying one for his wife - and then having her look at it and say 'Me? Sew? That is what I pay my seamstress to do for me!" And the machine got put in a room and never used.