Buying used or sight unseen speakers a bad idea?

Lots of good advice. I'd add that it is often reassuring if you can call the seller and talk to her or him directly. Explain that you're new at this, and that a conversation would help establish confidence on both sides. You're bound to get either a positive or negative vibe, which you can use in your decision-making process, along with the seller's feedback.

You might also search for posts from the seller in the discussion forums. If the seller is a jerk, that particular personality trait is likely to come through in hie or her posts. I've done quite a bit of buying here and never had a problem.
-Bob

Hesson11 has a good point. See if you can call the seller to talk to him directly. I had one fellow call me just to pick my brain, to see where my head was at, before he made his decision to buy. He asked why I was using this item, and why I'm selling it now. We talked about how our systems started and how they got to where they are now. What we liked, disliked, and why. He was comfortable with dealing with me.

I agree buying used is always a risk and yes, the sound can be degraded if the speakers are abused. I like to ask the seller if they are the original owner, if the speakers have ever been repaired and what amplifier was used with the subject speakers. I also make it a point to buy from sellers with all positive feedback. Even after all of that I recently purchased a set of speakers that arrived in very poor condition. Fortunately the seller took the speakers back and gave me a prompt refund. However, it cost me over $100 to find out the true condition of the speakers. A recent post here recommended observing how sellers conduct themselves on the forums. I think this is a good idea because it can tell a lot about a seller's character. This is good to know if you have a problem with a purchase.

Often the best approach, if possible, is to do a pickup and have the units demo'd. That way cost is lower since shipping itself is risky. Also gives you a way to check the surrounds, grills etec. I have purchased over six pairs of speakers this way, and yes I rejected three other pairs.

Seems like a lot of classifieds have appeared recently here on Agon with feedback scores of 0. Perhaps it's just due to the un-economy, and people are needing to raise some money.