The number one rule in any guide on how to buy a used treadmill is to not buy a treadmill sight unseen. I know this is difficult to do when shopping online, but there are many factors to consider before purchasing a used treadmill and they are difficult to review if you can't see the machine in person. When reviewing a treadmill, you must look at the following factors.
How much life does the motor have left in it? This is difficult to know for sure, but ask a lot of questions. How old is the unit, how often was it used, etc. Compare the answers against the manufacturer's specs of how long the motor should last. Also, if you can test it in person, run the treadmill continuously for twenty minutes. Listen for any change in sound or slowing of the pathway. This will indicate motor issues.
Treadmills
How well does the console work? This is a test that should also run approximately twenty minutes. Watch the icons for any blurring or fading. Also, test the timer against your own watch for accuracy. If the timer is inaccurate, chances are so is the entire data system.
How big is the running path? A lot of the older treadmills came with a pathway that was too short for many to run on safely or comfortably. A pathway 52 inches long should be the minimum. Also, watch the pathway while the treadmill is running. There should be little to no left to right movement. If there is too much noticeable movement, DO NOT PURCHASE. This could be a sign of serious problems internally that would be expensive to correct.