Is buying a “used” subwoofer a bad idea?

You have received thoughtful replies.  It is like anything else you might buy used, including a car.  Evaluate the seller.  Do they seem to be someone who takes care of their equipment and generally has their s__t together?  Also, is the sub a brand that is still commonly supported by the manufacturer.

I own three subs that I purchased used.  All of them are used almost daily for the past 5 to 15 years.  Two have list prices over $5K.  They all work and sound great.  One of them needed a repair to the amplifier that the manufacturer took care of promptly and informed me the issue was a design issue so I purchased a spare amplifier for the other sub (two of them are the same) in case I had amplifier problems with that one (original amp still works fine).  In conclusion, buying a high quality product, that is still supported by the manufacturer, and from a careful seller, is really no different than anything else related to audio that we purchase used, IME, 

@kennyc 

I have sold one individual and three pairs of subwoofers over the years because I did not like the way the integrated with my main speakers.  Absolutely nothing wrong with them nor were they abused in any way.  Not saying that does not happen but there are good ones out there.  Buy from a trusted seller. 

My KEF KC62 sub is hardly used, and I am going to sell it.

Buy one that you can still get repaired!

I’ve had my Sunfire Signature repaired a couple times and it is in the shop (EBC Electronics) right now, even though I did have to send it across the country. They have a fixed repair fee ($255), regardless of the problem.

For an additional $25 I opted for a 1-year warrantee because I figured they would have a "harder look" at everything if they had to guarantee it for a year (normally 90 days) and they get $30 for the return shipping.  I paid $36 to ship it there with insurance.

I think it went dead because the speaker got unplugged from the amp and Sunfires hate this.  The amp sits in its own ventilated cabinet and the speaker has a block-off plate.  I did this after the second repair job.  Sunfires will literally shake their components to death from the internal cabinet vibration.

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.