Is buying a “used” subwoofer a bad idea?

I\Hi Kennyc, I think buying used anything is dependent on several things. Some products are internet garbage while others are cared for. Is the buyers responsibility to decide.

That's BS in my opinion.  Anything can be abused and quality subwoofers have much stronger drivers in them to withstand longer excursion, lots of constant use, etc.  Most every subwoofer I have ever bought was used.  They weren't junk, but good stuff from Martin Logan, REL, Vienna Acoustics, etc.

The risk is greatly lessened by the fact that you’re (presumably) buying an expensive audiophile sub from an audiophile. Most of these subs are overbuilt for how audiophiles actually use then. Yes, if you were buying a thump cannon HT sub, car sub, or cheap sub from a kid - abuse would be a concern. 

How deep do you want your paranoia to run? At some point, you simply have to shift to buying everything new because you’ve talked yourself into seeing too much risk. I don’t run a sub but I’ve bought (for example) many expensive used MC cartridges (very delicate, wear items) from the audiophile community, without any problem. Check feedback history - especially as a seller. 

These days my main concern about buying a used sub would be shipping costs, ugh. Then again I did just ship 220 lbs of tube amp so it can't be much worse than that.

“Is buying a “used” subwoofer a bad idea?”

Not at all, verify seller feedback and condition of the item before committing to buy a used item. 

+1 @lalitk  Buying a used sub would be pretty low on my risk radar to start, and if you buy a quality sub in good condition from someone with 100% positive feedback I’d have no concerns about it whatsoever.  In fact I bought my sub used using these parameters and it’s perfectly fine.