I have gone the "used" route on e-bay; the key is to ask the seller questions about the item's usage - what kind of riding, for how long, etc. If the seller replies promptly and actually answers your questions, ask a few more questions that require a bit more detail. Another prompt and responsive reply, you probably have a MTB'ing seller who is going to be honest with you.
Most important question - especially of it's NOT addressed in the sales blurb - is why they are selling whatever it is. Did they win a new, better one? Did they get a new bike & they're parting out the old one? Is it worn & they simply don't want to deal with the rebuild? Or is it just (OMG) red?
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As with mud, life, too, slides by.