Jon Siegel replies: Are there parts missing? If so you should check on the availability of these parts before buying. If the machine is old, you may not be able to get original parts. In this case, consider fabricating the parts yourself or buying an aftermarket item (such as a table saw fence) which might be even better than the original one.
Look for signs of poorly made repairs. Castings which have been broken and brazed back together are rarely accurate, and may break again.
Has the machine been taken care of? Obviously, you should look for rust, or any signs that the machine has been stored outside. Have the adjusting screws been lubricated? If adjustments work smoothly, and the knives are sharp (jointer or planer), this is a good sign. If buying a machine from a woodworking shop, look at the shop itself. Is it orderly and well kept? Although indirect, this is an amazingly good predictor of the maintenance the machine received.
Joe Barry replies: With used machinery I like to see it run. If not possible, when was the last time it was run? Stick to the established brands that support their tools. I can still buy parts for my Unisaw built in 1948 from Delta. I just bought a used Powermatic shaper because I know I will be able to maintain it and get accessories. Check the manufacturer’s web site to see if parts are still available for the machine in question. If you want to buy a fixer-upper, ask yourself why. Are you doing it for the pleasure of fixing it up, or the cost? The cost and time will often exceed reason with making old tools serviceable. Most importantly, know what you are buying! Don’t buy a square head jointer or planer because they were considered unsafe in our grandparent’s time.