Is buying a “used” subwoofer a bad idea?

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. 

Buying any kind of used audio gears, regardless of price, is just like buying a used car.  It all depends on: Who you’re buying it from (does the seller have a track record of integrity and reliability?):  Where you're buy it from (will the seller be held responsible for selling a not as described or defective item) and, of course:  How the item was maintained buy the seller (seller provides an accurate and detailed description of the item with photos attached).  All critical. All the above lessens the risk.  Happy listening.            

If you can verify it is an audiophile that is selling it... as opposed to someone that resells garage sale stuff, or buys trade-ins from audio stores. You can probably feel very comfortable.