Good and bad experiences will depend far more upon the individual seller than they do upon eBay itself. I have purchased 20 or so pieces over about two years' time off of eBay, and almost every auction goes well. My few problems were very minor and worked themselves out quickly to my satisfaction.

Make sure you check out the feedback rating of the seller before you bid. Ask all questions about condition, edition number, variations, signed/unsigned before you bid. Also, make sure you are educated about the value of a piece so that you don't get taken for a ride.

Know in your own mind what a piece is worth to you and do not get caught up in the bidding and exceed that price. Usually, there will be another piece of the same model that comes along within a month or two, so you'll get another chance. Don't chase the price up. Be patient.

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-Art