Is buying a “used” subwoofer a bad idea?

Depends on how much it's been used. A light use subwoofer on a 2ch audio system -versus- a subwoofer constantly hammered in a dual purpose audio system and home theater setup (combined use) is a used subwoofer I'd avoid unless you are a fan of the cabinet, get it for low cost, and don't mind rebuilding it later.  

Having helped multiple friends replace the woofers and amplifiers in factory made subwoofers and there is no guarantee a high grade sub will last longer than a low cost one. Seen and helped fix some nice subs that did not last long and then the factory parts were no longer available so it was a total cut / fab / do over.  

Can also be rewarding to build your own subwoofer kit with a new amp, new woofer. If the amp or woofer goes bad later, it's easier to fix since you already know it having built it. 

I bought two used powered subs - the same model - at different times - over a decade ago.   Eventually, the plate amps for each died years apart.  No replacement plate amps would fit in the custom cut-out in the cabinets.   

Since I loved their design - and sound, I’ve kept the subs going with a standalone sub amp - a new Dayton Audio SA1000.    Since the Dayton has more fine-tune settings than the original plate amps, my subs integrated even better than before.

Sure enough, the Dayton sub amp died after 4 years.  Dayton customer service would not even consider repairing it.  A web search yielded that the problem might be just one particular capacitor (C-6 position).   Being handy with a soldering iron, I fixed it myself for $2.   It’s been working great since.  

I purchased two REL S3 SHO subs and "long bow" wireless units second hand from different sellers. They have been perfect for last three years. When they are set up properly they really don't work that hard. If you can hear them you have them too loud/wrongly set up. Mine are now used wired to the Tube amp speaker outputs to the hi-level inputs of the subs working alongside but positioned inside my Proac Carbon Pro 6.The system sounds wonderful. So do your due diligence, talk to the seller about their experience, what they are used for, how they are used and set up etc. Walk away if you have any doubts I tended to avoid ones used in home cinema systems, just because their role seems to demand more work than in a two channel stereo Hi Fi system.

“When they are set up properly they really don't work that hard. If you can hear them you have them too loud/wrongly set up.”
 

@nubiann - That is a really good point. The other day, I shut down the amps to my main speakers in order to change some cables.  When finished, I started to play music but had forgotten to turn the amps back on so just the subs were playing.  Considering the size of the drivers and amps in those subs, it is a bit surprising how little output they contribute. However, if you turn them off when your system is playing music, they are certainly missed.

That’s precisely my experience too. I am lucky to live within 20 miles of where REL make their subs in Bridgend Wales. I have enjoyed having someone help me set them up optimally. The result is exactly how you describe never aware of them until you swicth them off.