Humphry - All of the above posts have mentioned some very valid points to consider. Personally, I've probably had only 7 or 8 sets of speakers over the past ten years; and can't honestly think of any problem I've ever had with any used speakers I've bought. In fact my Thiel 3.5s, which I bought from the original owner, are about 10 or 11 years old and sound better than ever. (Partly due to better sources and electronics over the past few years!)
Mostly what I would be concerned with is the past user's treatment of the speaker. Does he/she play music at high levels? Listen to lots of rock/rap/heavy metal? Do the speakers look like they've had a rough life? (Probably a good reason NOT to buy them!) Let's face it - in this hobby of ours - condition counts for a lot! Do your homework, and by all means if you're going to make a speaker purchase listen to them BEFORE you buy. Just because a speaker is highly rated by TAS or Stereophile doesn't necessarily mean that it will sound great with your equipment. Good listening!
Mostly what I would be concerned with is the past user's treatment of the speaker. Does he/she play music at high levels? Listen to lots of rock/rap/heavy metal? Do the speakers look like they've had a rough life? (Probably a good reason NOT to buy them!) Let's face it - in this hobby of ours - condition counts for a lot! Do your homework, and by all means if you're going to make a speaker purchase listen to them BEFORE you buy. Just because a speaker is highly rated by TAS or Stereophile doesn't necessarily mean that it will sound great with your equipment. Good listening!