All of the above. Plus, if you're not sure about anything at all, ask the seller. If they're genuine, they won't have a problem answering questions, and if not and they say it's fine when it isn't then you have evidence that they misrepresented the item. With an SLR, ask if the shutter speeds and the meter are accurate. With lenses, ask if the lens is clean, and if they just say it 'looks clean' ask them to look at the inside of the lens while shining a flashlight through it - remember, to the seller, 'clean' might simply mean 'no big chips or scratches'. Sellers may not be intentionally trying to con you, but many may be selling the items for other people or otherwise not know anything about photographic equipment.
Always read the small print. Example, one listing I was recently listing stated that "the meter works and has not been tested with film". I would have thought "the meter works
but has not been tested with film" would be more appropriate; that seemed a bit sneaky to me, as if trying to fool people who don't read it properly that it had been tested. Of course, that too could have been an innocent mistake. In any case, I'd be especially concerned about anyone who refuses to accept returns, unless they specifically state that the item is faulty. It's important to make sure the seller will accept returns (ones who offer to refund postage costs are rare but obviously even better) as
eBay and Paypal will be no help whatsoever. It seems
eBay's attitude has always been and will always be simply "most people don't have any problem with eBay", which I've always thought is a bit like the police refusing to catch muggers on the grounds that "well, most people don't get mugged".