I am a lot more likely to buy a used component than speakers. My last few speakers have been new , reason for that is my last pair of used speakers needed a part that took months to get... the seller was kind enough to pursue the warranty but it took 4 or 5 months.
The only used speaker in my experience that is a safe bet are speakers like Heresy, Forte, Cornwall.... if you have a problem they are easy and relatively cheap to fix.
Best bet is a speaker that isn't too old and has a transferable warranty, but not too many speakers have transferable warranty... ATC is the only one I know of. Maybe there are others?