The Latin phrase is Caveat Emptor...Let the buyer beware. Ebay, especially so.
I've been 'taken' a couple of times on ebay, but that was in buying used computer parts that were indicated as working and in one case, a too-good-to-be-true "genuine" copy of Microsoft Office. It was a copy alright, from the sellers CD burner.
As for buying photography equipment on ebay, I've never run into any problems. OK, I bought what was sold as a Canon BG-E9 grip for a 60D for $45 or something like that, from China. It was a good fake, and worked very well, so I wasn't disappointed. The price was too good to be true for genuine Canon, but I didn't know better at that point. I've bought two used Canon L lenses on ebay, both very good deals. I made sure it was a reputable seller by looking at feedback for the past 6-8 months or so, and checked the pictures very closely to see if there were any scuffs, dings, etc. I especially want to see the condition of contacts on the back of the lens, to see if the lens has been on and off the camera a zillion times or perhaps 100-200 times. Of course, the description should also state no fungus, scratches, etc. One seller was a camera store in Japan. The 80-200 f2.8L I got from there was as close to mint as there can be. It works perfectly. My 200 f2.8L came from a seller at a small company that had perfect feedback and had sold some camera gear in the past, as well. When I moved from a point-and-shoot to a used 30D and 2 lenses (3 different sellers), I did the same level of due diligence to make sure my $300, $400 or whatever wasn't getting ripped off. Lesser priced stuff, $50 and under, mostly, other than 'dumb digital rookie photographer mistakes' like buying screw on macro and telephoto converter 'set' for $35 or so, have mostly had OK results. But over $50, I start looking quite closely at the sellers reputation.
The same goes for online stores with impressive web sites such as "Big Savings Camera Depot" (made up name) that nobody ever heard of selling new Canon 5D3 cameras for $500 less than anybody else, etc. Those are guaranteed to take you money and never be heard from again. Or they ship you other than what you bought and paid for, or you wait MONTHS to get it...
And, for what it's worth, I've found that ebay is OK for selling used camera gear, as I have. But their 9% BITE out of the wallet for sellers fees surprised me big time when I sold my 24-70 f2.8L about a year ago. I also have a perfect rating as both seller and buyer.