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Lower quality models generally have a shorter life expectancy, so buying one of these used is less than ideal. Beyond that, trusted brands develop their reputation through the strength of their products, so it's more likely one of these will hold up better than a no-name brand.  Some highly regarded models include: AFG, Gold’s Gym, Horizon, Landice, Life Span, Livestrong, NordicTrack, PaceMaster, Precor, Proform, Sole, and Vision. If you find it difficult to choose the best brands to hunt for, you might ask a person at your gym who is knowledgeable about treadmills. History matters a great deal when buying a used treadmill. Don't hesitate to ask questions about its usage, how well it runs, and features the seller liked and disliked. After all, even with a good deal, it's likely you'll be investing a good chunk of money in this purchase. Generally, ask questions to find out how much the treadmill has been used. The less it's been used, the better, as this means there will be less wear and tear on the machine. Since you'll be running on the treadmill for exercise, it only makes sense to give it a test run before you buy it. But first, be sure to ask sellers if they'd mind you giving it a spin. If a seller refuses you a test run, this is a huge red flag and you may want to consider buying a different machine. Avoid hopping on and hopping off the treadmill. This won't give you an adequate experience. Give it at least 5 to 10 minutes to see how to it feels. Look the treadmill over from top to bottom and front to back. Pay close attention to anything that looks broken, cracked, or worn out. This could be evidence that the treadmill was dropped or mishandled, which could harm its internal performance.  The most important feature when looking for wear and tear is the belt. If you notice fraying, tears, or if it looks worn out, seriously consider a different model. Cosmetic damage, like a chipped cup holder, won't effect the performance of your machine very much, but you could use it to help negotiate the price even lower.
Prioritize highly rated models from trusted brands. Learn the history of the machine. Test out the treadmill. Locate wear and tear.