I'd stick to the big camera-first dealers like B&H and Adorama, and to well known regional camera stores that sell online. The two in New England are Hunt's and Green Mountain Camera - I don't know who the equivalents are elsewhere.
Even big non-camera retailers like Amazon have their problems, since they carry so many products (most of which are much, much cheaper than a professional camera). Some poor photographer ordered a 1Dx mkII on Amazon (shipped and sold by Amazon itself), and got a Canon box - with rocks inside. He returned it, and got another Canon box (containing a brick)... Fortunately, the guy videotaped the second box opening starting with the sealed Amazon box, so Amazon gave him his money back. His subsequent B&H order actually contained a camera!!! Someone working in an Amazon warehouse had presumably stolen the cameras and replaced them with ballast. Since B&H deals with so many valuable photographic items (and is much smaller), their warehouse is better monitored...
The second problem with Amazon, eBay and other large "sell everything sites" is the large number of third-party fly-by-night sellers that look just like real Amazon listings. Gray market cameras, third-party batteries and other "equivalents" are not always clearly labeled.