This is an interesting thread...
While "brand" might be a good starting point (like all those listed already), I'd first look for compoments and frame type. While we may all dream about finding a Campy equipped bike for $20, realistically spotting nice Shimano or Suntour components is a good sign, along with alloy rims and a nice seat.
I've noticed no one has listed Cannondale. While I'm not a huge fan of aluminium frames, Cannondale made some very nice bikes that are now 20+ years old and may be entering garage sale territory. Go back 30 years and add Motobecane and Raleigh to the list, too. Of course, I'd have to snatch up any old Austro Daimler!
Have fun shopping!