Others questioned the same thing when Adorama did it via eBay for $2749.00 USD. That's when I got mine and it came new in box, serial numbers matched, no light leak issue and I've been shooting with it for almost a month now and absolutely love it.
I don't know why dealers are doing it via eBay, but as long as its not Gray market and a reputable dealer, I say go for it for those that want to.
I guess eBay is the place to help those Canon authorized dealers to sell the 5D3 without "official" promotion. Since it's not an official promotion for 5D3, Canon cannot complaint to those authorized dealers selling 5D3 cheap.
After the BeachCam 5D3 popped up for $2700 a month ago or so I called them. I said "Hey, I have a question about a product you are selling on eba---" He interupted "The 5D Mark III?" He said they had been getting calls about it all morning. All he would say is that they Canon gave them a discount on the order and it was a special deal.
I dunno what that means, or how the relationship between Canon and Authorized Dealers works exactly but that's what they told me.
So I order, and received one.