deals the dealers don't even know about?

You can buy anything you want on the internet, from your home and get bargains that even..
As for dealers, this is kind of a vague term, but lets just say the middle men. The companies that buy goods from the manufactures and sell it to the retail stores and so on. Dealers though has a kind of shady feeling to it, like drug dealers. It is to my ear at least, sort of like a guy who sells stuff out the back of the truck.
So, the above statement, means that if you buy stuff off the internet you're getting a steal of a deal, or the best buy possible.

But can I take it as "everything you can imagine"?

absolutely