Sorry I did not address the second part of your question. Glide bar clamps do not contact the full surface of the glide bar. Inside the clamp where it rides on the bar is a small bushing built into the clamp. Constant use will cause metal to wear. If I were going to purchase a glide bar machine (I'm not saying there is anything wrong with them) it would not be a used machine nor would it be one of the lower quality machines no matter how good they appear to be at first.
I would like to apologize to anyone I have not offended. Please be patient I will get to you shortly.