There is this store I frequent, that sells magic, world of warcraft and yugioh, and I had to go to wally, or travel more than 6 miles to a store that actually sells pokemon cards. The good thing is, they started selling pokemon cards, and they noticed that they sell out fast. The problem I see with their store is, they only have room for 1 theme deck box, and 4 booster boxes, so space is a huge issue. I guess Magic, Yugioh, and Pokemon is probably the best bet, since they are the 3 most popular TCGs. That same store also sells board games, like catan, ascension, pandemic, and the like. They also have an area where people play TCGs, and computers for lan parties.
I am not a detroit guy personally, but I heard of this store in Detroit, owned by some dude named Steve? You know, the guy who makes funny youtube videos to promote his store, ahem, website? Usually, when you have a lot of space, like his store, you are free to sell whatever you feel like, such as TCG cards, sports cards, and sports memorabilia.
To run a store, you better buy a crap load of product, because pokemon packs sell like hotcakes. I witnessed it firsthand at my local store, because they were starting to get into the whole pokemon thing, so they wanted to test out whether or not they sell or not, and boom, 3 of 4 booster boxes, one BW, one EP, one NV, and one ND sold out, leaving only BW boosters.
It also depends on where you are located. You don't want to be competing against stores like that Steve store I described above.