Remember the fundamental rule regardless WHERE you buy from (even from your mother): "Caveat emptor" - "Let the buyer beware"
As per "Spotmeter" highlighted: Ask LOTS-n-LOTS of questions. With eBay, the number of positive feedbacks is generally a good indication coupled with paying via PayPal - you are quite safe. When/if things go pear shape, paypal will resolve the matter BUT don't expect quick resolution - thus the only down side of it. At the end of the day, you need to factor in your "calculated risk". I have sold/purchased a lot on ebay and so far only had ONE bad pathom auction for a baby bath (won auction but seller sold to other at "buy now" price) - got money back and ebay seller stuck off! (but then again, s/he can recreate a new one with a new bank a/c - BUT will have to start feedback afresh from zero - not somethin you want if one is going to sell a lot on ebay. Lastly, I would hestitate purchasing $20K MFDB notwithstand ad says "sold as is in excellent working conditions". Good luck!