I've bought and sold on Ebay, no problems. Like the suggestions above though, try to deal with people with high and near 100% feedback - it doesn't mean you will get a great item at a great price, you might be unlucky but the really poor sellers are weeded out from Ebay reasonably quickly.
Check what you read very carefully, ask lots of questions and find out if it is sold as working. If it's sold as working and arrives not working, then if you have had some communication and been told it is working then you have at least some comeback.
You always run the risks though, let the buyer beware is a common phrase. Avoid overclocked stuff like the plague and if a deal looks too good to be true, it probably is. Be sensible though, deal with reputable sellers and you are likely to come off better than had you gone and bought new.
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