When people ask me "which exercise machine should I get?", I have a standard answer.
First, never buy an exercise device at a garage sale. If it's up for sale, it's probably been underused (except perhaps as a convenient place to throw dirty clothing at).
I advise shopping in a store that sells such equipment. When you go in, look interested, so a salesperson comes over and smells a sale. When the salesperson comes over to you and says the inevitable "May I help you?", respond "yes" and watch him start salivating Now you've got 'em hooked! Your initial question should be the following: "Does this store have a 30-day money-back guarantee on all the exercise machines that you sell?"
If the salesperson avoids your eyes or starts mumbling "Uh"" or is otherwise evasive, let him know that you're "Just looking" and leave as soon as possible.
But if the salesperson smiles, and shows you the policy in writing, then buy anything you want at that store. If it doesn't work out, you know you'll get your money back. Rather than having the device end up as an expensive place to throw dirty clothing.