True story: In early 2007 I bought a relatively complex piece of equipment (a special type of scanner). At the time it was a new technology that not too many people had access to and which required a fair amount of technical knowledge to operate, but I had one in a spare upstairs bedroom and it was in demand. I originally bought the machine for my own use but quickly figured out I could make a little extra money on the side running jobs for others. I referred to it as "my expensive hobby" at the time because I was still working for someone else full time, until one day I sold a large contract that let me quit my day job and run that machine for six months straight -- enough time to line up the next big project.
Today, I have 30 employees and what most folks would consider to be a very successful small business. However, I don't run the machines myself anymore. :-) I was fortunate to have one of my interests intersect with an industry which will always need related services. I also have a very supportive spouse who could have shielded me from the "what if this doesn't work out?" scenarios if one of those had come to pass -- at least until I found another day job. And of course, I had a bit of luck. So yes, you can take something that you're incredibly passionate about and turn it into a business, but it's a ton of work. Also, I had specialization on my side -- focusing on providing one type of niche service that's in demand, and doing it better than anyone else -- and still do.
What would make yet another hobby shop different from all of the others? I get a dozen emails every day (at least) from various online train retailers. Besides lowest price, if I can get the same box from any of them, what makes me buy from them? Support? Reputation? Their website? Etc. Retailing is ultra-competitive. I'm certainly not saying that it can't be done, but it takes a ton of planning.
I'd echo what others have said above -- it's a lot cheaper to buy or build a building and fill it with trains for your personal use than to try to build a successful hobby retail business with some display layouts in it. Probably a lot more fun, too.
