Or, as boat owners say, the two happiest days of your life are the day you buy it and the day you sell it.
Dave Nelson
It would seem to me that having fulltime employment plus running your own fulltime business would be pretty hard - if not impossible!! In my area, there are several guys who have sort of ‘semi-pro’ hobby shops, they are open like one day a week out of their basement or other part of their house. A lot of their money comes from selling at flea markets and train shows in the area. I’m sure some of them do it just to help subsidize their hobby - get stuff at wholesale price from Walthers or others. The ‘semi-pro’ option might be more realistic than renting a shop in a mall - you have to think about staff (assuming you’re not going to be there FT and at your job FT), maintenance on the building, heating and A/C, insurance etc. once you get a shop running.
p.s. You may not be able to just stock “best selling items”, I’ve heard LHS owners complain about how to carry certain companies products you have to carry X amount of what they choose to send you. Like to carry a certain magazine you might have to buy a minimum of say 50 a month, you might only sell 10 or 20.
I think the easiest way to ‘dip your feet in’ is to be part of the problem - start an online business. It won’t necessarily put you behind the counter, but the drill of figuring out what comparative advantage you can being to the hobby is a lot cheaper on the Internet. There are all kinds of great MR businesses out there that do something very well - Fast Tracks, for handlaying, or any number of brass dealers.