Had my Martin Logan sub for about a year now, it was used, no issues. Think anything you purchase used can be a leap of faith. Think the higher end you go, the more likely it was taken care of, or used responsibly.
If you are the type that looses sleep at night over a warranty expiring, then stay away from used. Or if that used piece fails and you do not have the means to sort it out. Then just buy new, without any worries.
For me, I'm on a budget, but I like nice things. A $3k sub is not in the budget, so I get a used one for a fraction of the price, I'm happy to have my new to me sub, I keep something out of landfill, also keep some money in my account.

