Generally speaking I would recommend that you stay away from the low-end treadmills, such as a lot of the ProForm and NordicTrack treadmills, etc..
Often people will purchase a treadmill with good intent. They use it once, or twice, or maybe even a few times. Soon it falls by the wayside, like so many diet and exercise plans, and it becomes a place to drape clothes. These types of treadmill can be a great deal if it is a good quality treadmill.
Here’s what to look for:
1.) How old is the treadmill? I recommend staying away from treadmills that are more than 5 years old. Especially in the mid-range to low end. Many parts become obsolete after 10 years or so. An exception to this is if it is a high end treadmill like a Landice or other high or commercial treadmill.
2.) Does the treadmill look new or like it is hardly used? If the belt has two shiny tracks running through the center of the tread belt when one would walk, then this is a well used treadmill. You may be buying someone else’s problems. The belt and/or deck may need to be replaced. This can easily add a few hundred dollars to the cost of the treadmill.
3.) Look at the size of the rollers. Inexpensive treadmills use small rollers to cut down of the manufacturing costs. These require that the tread belt be tightened more so that the belt does not slip, since there is less surface area on the roller to grip the belt. This puts more stress on the belt and roller bearings shortening their life. I would not recommend purchasing a treadmill with rollers that are less than 2½ inches in diameter.
4.) Run on the treadmill for a bit (don’t buy one that you can’t try it out). Does it start up and operate okay at all speeds? Does the incline got all the way up and down? Do you hear any bearing noise in the rollers or motor? Bad bearings in the rollers may sound like a rattle or grinding noise. Bad bearings in the motor can sound like a high pitched or electrical whine.
5.) If you can, loosen the tread belt so that you can slide your hand underneath and feel and visually inspect the deck. If the deck has grooves in it, or is the black is worn through exposing the brown underneath, then the deck, and possibly the belt are worn out. Note: You should loosen the belt by loosening the two rear roller adjustment bolts equally about 8 turns counter clockwise. Then tighten them by turning the exact same number of turns that you loosened them by turning the bolts clockwise. These are typically allen bolts and the owner will often have that tool handy. Sometimes they are standard hex bolts.
6.) If you can take a look under the hood, that is ideal. Often motor covers are easy to remove with just a few exposed screws. However, sometimes they can be a real pain requiring that the treadmill be folded up, and then laid down in the folded position to get at the recessed screws that hold the motor cover on.
If you can see under the hood, look to see how much dust, dirt, and pet hair has accumulated. You can get an idea of how old the treadmill is, and how well it was maintained.
Inspect the drive belt for age or wear. Is there a lot of black rubber powder under the belt from wear?
Horse Power is not so important as is motor size and quality. For example the small motor to thee left is a 2.65 H.P motor out of a ProForm 520X treadmill.
The larger motor is an 3 H.P. American made Baldor motor out of a Landice L7 treadmill.
The larger motor is continuous duty, the smaller motor is not.
7.) The size of the motor control board is also a good indicator if the treadmill is a good quality treadmill or not, but obviously not the only one.
Many inexpensive treadmills have very small underpowered boards that have a circuit board card that is only about the size of 3X5 index card. These will do fine often for several years until the belt and deck get too worn and/or are never lubed that the board blows out because these inexpensively made boards just can handle a 12 amp draw being pulled through them and the burn out. This is very common on lower end treadmills. In fact it is the number one cause of failure in lower end models. On the other hand it is a rare thing in my experience to see a board blown on a high end or commercial treadmill that can often withstand 30 amps.
Personally I would prefer a high end or a commercial treadmill even if it needed some repairs, than a low end treadmill in like new condition. Keep in mind that many treadmills coming out of a fitness center or gym are heavily used and not well maintained. However, if you can clean them up and perhaps have a deck that has not been flipped yet you can have treadmill that may provide many years of great use by simply cleaning it up and maybe replace the belt and/or a set or two of roller bearings. Keep in ming that commercial treadmills do not fold up. If you have the space, then that is an asset as non-folding treadmills are much more stable.
Lastly, be careful who you listen to. I just read an article about buying a used treadmill by some so-called Health and Fitness Experts. Several of the brands that they were recommending were of poor quality such as Livestrong, ProForm, and Gold’s Gym. One brand that was recommended (PaceMaster) has gone out of business. They did supply some common sense advice, but was obvious that they did not really know treadmills.