Rule of thumb is that...
- if the price is too good to be true, it usually means stay away
- if the price is lower than what it usually retails for in your country, it usually means its a grey market (real camera just that it's being sold in a different region than it was intended for. ie: a camera that is meant to be sold in Asia/Europe but is being sold in the US)
- ALWAYS take a look at the feedback to see what their selling history is. Sometimes, these sellers can sell small items or buy stuff just to build feedback and once they build enough, they sell higher priced items. Some accounts can be stolen (via phishing), items listed for sale and when the original owner finds out, eBay takes actions by closing the account and suspending all sales.
I actually bought a FitBit for my wife on eBay. Price was a great deal but I noticed that location and names on eBay and PayPal account did not match. I did however receive the item. Went to leave feedback and account was closed. So I got lucky.
Having done over 2000+ transactions myself, you start to figure out and know what is legit and what may not. I've seen some listings where it says its being shipped from CA but the seller is listed as coming from Indonesia or some other country with minimal feedback. They may be legit but I wouldn't know that until I buy from them but something I would be cautious about.
I've heard Nikon will not repair grey market gear but Canon will and has repaired them.