Is buying a “used” subwoofer a bad idea?

Hope not.  The one I’ve used for the past 4 years is used.  

Buying a good used sub can get you more performance for the dollar.  Obviously it depends on the particular woofer and the usage.  Some handle abuse better than others, and not everyone pounds on them.  I sure don’t. 

Subs can be a very safe buy...

Audiophiles will rarely bottom out drivers and do stupid stuff with subs. You have to be a li'l careful with some younger hometheater dudes who go crazy town with subs though. But, the type of subs they have won't be the type of subs that audiophiles tend to get...not much cross talk there.

Worry more about buying certain amps and so on from audiophiles who leave them on all year long. Audiophiles tend to get stupid in other ways than the home theater dudes.

My issue with a used subwoofer is the built in amplifier. 20 year old passive speakers? Generally ok and condition is easier to verify. 20 year old subwoofer amp? Could die at any moment. 

My Velodyne SPL 1000 is now 26 years old and still pumping bass (in a small package). 

Buying a used sub would depend on many factors. I think buying a used REL or similar is probably a safer bet than an old Big Box purchased sub from a lesser brand.

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,