Test your used treadmill to make sure it's in good working condition. Usually you cannot sell used treadmills that don't work properly for as much money. Also look for the user manual, if you still have it.
Go to the Play It Again Sports website (see Resources). Find locations of the used sporting goods and fitness equipment store near you. You can also look for any local stores in your area that buy used fitness equipment. Take your treadmill, with any accessories and user manual, to your local Play It Again Sports to sell it to the store. You can trade in the treadmill for other equipment at the store or you can get cash for it. The price you will get varies depending on the condition, brand and demand for the treadmill.
List your treadmill for sale. Place an ad in your local newspaper or online on sites such as Craigslist, PennySaver USA or eBay (see Resources). Some sites allow you to post photos of the equipment. In the ad, give the brand and model of your treadmill, as well as the approximate age.