The steep discount is fishy. When something is too good to be true, it usually is particularly on CL. All you really need is pics of the box, invoice/receipt (look at date of purchase), and paper work for the warranty prior to going to her place. Without all three, I would question her hard. If it indeed has a warranty, I don't feel a need to test onsite.
Another thing to be aware of, treadmills are HEAVY and long. The motor in those things weigh a lot. My roommate and I had one hell of a time getting ours upstairs.