You have to have your wits about you when using eBay. There is good value to be had, but equally there are many traps to fall into. Some thoughts / experiences:
- always use Paypal. I've previously lodged a successful claim with Paypal in respect of a lens that had fungus. Got a full refund. I think Paypal cover up to $1000, but only one or two (?) claims in any calendar year. Also be sure to always ask explicitly if a lens has fungus or scratches or anything else wrong with it, if not already stated in the listing; that way you have a more clear cut case if the product does turn out to be less than perfect.
- only buy from sellers who have a substantial positive track record. Ideally 100+ with 100% positive feedback for private sellers, and 1000+ with >99% positive feedback from traders.
- beware of zero feedback sellers with a high priced item for sale at a low price... they're almost certainly scammers... for example this Canon 1D4 with a "buy it now" price of $985 that sold yesterday... I hope the buyer is located in Darwin and will go to pick the item up in person for cash; wouldn't fancy his chances of receiving the item by mail...
- don't buy memory cards off eBay - go to "cheap as chips" (AP sponsor) instead. I bought a Sandisk CF Ultra III card off eBay at a great price... realised it was fake when the performance wasn't up to scratch. I eventually got my money back from the seller. The card was a beautiful copy - I showed it to various AP members at a meet-up, alongside a genuine card, and more people picked the real card as the fake... There's guides on eBay on how to pick fakes, but better just stick to cheap as chips.
My overall guidance would be the old "if its too good to be true, it probably is". But overall, although I've had some close shaves, I've never ended up out of pocket through using eBay (I've sold / bought around 100 items over the years, including maybe 10 camera items). The existence of eBay is, overall, a great thing as far as I'm concerned: it gives the used camera gear market "liquidity". But buyer beware.