Right after WW2, a lot of Lionel authorized dealers (reading the lists, as has been noted, many were appliance dealers or hardware stores) were listed in the books that came with trains. Do you suppose that that's because instead of store owners' looking for trains to sell, the Lionel Corp. reached out to them, and signed them up into what was to become the dealer network? Appliance guys and hardware store employees were probable a good bet to be established not only as dealers, but as a repair network.
Back then, there were three manufacturers of note: Lionel, American Flyer, and Marx. (Speaking of the toy segment of the larger train hobby) Didn't those companies have middle-men/salesmen who travelled around the country, contacting the stores and making deals with them? Travel was probably cheaper than a phone call back then, and if the traveling salesman represented several different lines, he could be successful and help all those stores be profitable. He couldn't email photos of the products and fax a sales document. It was legwork that brought success.
In the current climate of internet sales, and the plethora of manufacturers, I doubt if any of them are eager to support a network of small dealers other than the few already established big guys. How would the wholesale pricing compare to what is offered to the established stores? How much product would a new guy need to commit to? Draw a circle of say 50 miles diameter around your proposed location. How many model railroaders with disposable income are in the circle? Fifty miles is an hour's drive, generally.
I can't remember the last time I bought an expensive item (trains, sporting goods, automotive parts, tools, etc, etc) by walking into a store. It's just too convenient to "ask the Google" to find the product and click. Amazon promises 4-5 days, and the next afternoon, more often than not, the box is on the front porch, with free shipping.
Considering the MSRP of many of today's trains, I imagine that a small fortune is needed just to assemble a reasonably diverse inventory that would attract a client base and keep them coming back. "We can take your deposit and have it for you in a week or two" is not what it used to be before the internet.
One anecdote, for what it's worth: A small train shop opened in Riverhead, NY, 20+ years ago. Just trains, a few plastic car and boat models, some model plane supplies and a magazine rack. I figured, this is great...let's go in, introduce myself, offer encouragement, and buy at least something. "Grumpy" (I don't know his real name, because when I offered mine, he didn't reciprocate) took my $5.00 for a Garden Railways Magazine that I really didn't want, and turned his fat asset back around to continue watching the TV in the store. Not a "howdy" or "thank you" or "come again" or "what sort of trains are you interested in" or any other vocal interaction. The store was gone the next time I was in the neighborhood, a few months later. I would love to hear the whole story about that store.