Some people think that having carpet will allow them to get away from having to buy a treadmill mat. There is a lot of problems in this case because even if the carpet was enough to provide the same amount of cushion as a mat, the carpet houses a lot of dust and loose carpet fibers.
Buying a used treadmill is not unlike buying a new treadmill. There are things that you would look for and want in a new treadmill and there is no reason why anyone shopping for a cheap used treadmill should have to compromise those features and functions if that is what they desire.
The best treadmill mat will help protect your floor against damage and wear as well as the treadmill machine.