These are only Resellers using Amazon as the intermediary. Personally I do not understand how anyone can sell goods at so much below the regular price and I am extremely wary. However, look at the rating of the seller, and email the seller and get any further facts that you may be concerned about, like warranties, after sales service, authenticity, and so on. If you are happy with the answers, and are willing to take the risk because of the extremely low prices, and can accept the word of the seller, then it is still a gamble. I believe some people have sent money to such places and never heard from them again, but these people have been veted by Amazon and may have that little extra credibility.
Personally, for something like a DSLR, a life time camera and investment, I would stick to Regular places where you have recourse. There are still some bargains to be had, but smaller bargains, perhaps, but you have peace of mind.