Buying used or sight unseen speakers a bad idea?

Most high end speakers will last very long time without requiring repair or maintenance, so unless the owner has abused them buying used should not be a problem, but strongly recommend you at least audition the model you're interested in at trade show or dealer. Very risky to buy speaker based on review only and I personally would never do so, especially in the high price range.
Another great option, depending on where you live and dealer availability, is to buy a floor model that dealer is selling. Usually this will still have warranty and can be had for half retail price. Many of these can be found on audiogon as well.

As others have stated, buying used is always a risk. I bought my speakers from a guy in Hawaii and they arrived at my home in Florida in pristine condition. I've only had one bad experience here but I have to take part of the blame because I waited a few weeks before I tried it out. The blatant lack of communication from the seller told me all I needed to know.Oh well, live and learn. Good luck with your purchase, if you choose to make one.

I can mention one thing that has not been mentioned. There are knock offs of speakers recently that are being sold as new and authentic. This used to be a huge problem back in the late 70's / early 80's where people would sell them out of vans. Watch out right now for Harbeth knock offs. For more information visit their web site and read Alan Shaws information about it. Harbeth's are some of the most well reviewed and regarded speakers in the industry.

You might also find fake knock offs of Dynaudio, Sonus, Proac etc.

It is almost inevitable that you can't hear/audition speakers before buying them due to the continuing dirth of local good audio stores. The shows can help but require travel and audition under less than ideal circumstances. I have managed to buy just a few speakers that I didn't get a chance to hear but have bought some. Only a couple of those really dissappointed me.