Probably the Beseler 45 enlarger for $7.50 at a local school district surplus auction. I was the only one who knew what it was but some old woman still bid against me a couple of rounds.
There was the one that got away: an estate auction with a B&J Grover monorail 8x10 with a couple of old wooden film holders and three or four old lenses (not good lenses and not particularly valuable.) I had it at $60 when some guy ran up out of breath just before "Sold!" and bid. Irritated the heck out of me. I got him up to $500 and let him have it. He came up to me after the auction; he thought it was a camera but wasn't sure. He asked me whether or not they still made film for it! I pointed out the rail clamp and told him to take care of it, that was the most important part because it was often missing. Never did hear how that turned out.