Hi, my advice here would be to avoid any listings that say "I know nothing about cameras". Not only may the seller be genuinely unable to tell you about the condition of the equipment (which is not good in itself) but it could also mean they know it doesn't work and that's their cover. If the listing does not specifically say that the equipment is not working or is in poor condition, then ask if they will accept a return.
In the case of cameras, ask if it's fully functional; if the shutter speeds appear accurate, if it's a modern auto camera ask if the flash works and the LCD isn't faulty, etc. Ask about general condition too, i.e. is there visible marking or dirt in the viewfinder or other internal parts? If there's a lens, ask if it's clean; does it contain fungus? Does it stop down correctly at all apertures and is the focusing smooth? Of course this sort of information should be provided in the listing, so if not you should check; then you're better covered if you discover a problem on receiving the item.
Finally, don't buy a very expensive item (especially a digital camera) without checking the seller's feedback, but also their previous sales. If all of their previous sales were of goods costing $0.99 and all their feedback seems strangely similar, yet suddenly they're selling a Digital SLR, there's probably something wrong there. I personally noticed an incredible number of listings for pre-owned Canon DSLRs, all with an identical photo and identical text offering an unofficial buy-it-now price, all from sellers who previously sold suspiciously cheap/fake-looking tiny novelty furniture or cosmetics. In other words, obvious scam. Not all scams are that obvious, and so my advice is to get assurances - about what's included in the listing, the function and condition of items, the cost of postage, and the seller's policy on returns.
I don't want to make it sound like buying from
Ebay is particularly unsafe - chances are the majority of your transactions will go without a problem - but it's still best to avoid those few instances where you do have a problem.