I contemplated opening a brick and mortar supplement store about 2 years ago. I live in a small community in the Midwest, and there was only one GNC within a 30-40 mile radius of my proposed store. Lucky for me, I opened a gym instead, which has been a surprising success. I've added a small supplement section to it. They've sold well, but there's no way I could have made enough to cover my expenses and make a profit if I just sold them.
Since then I've expanded online with a membership type supplement club similar to Costco, which also has been doing great. I just wish Biotest would release more products for distribution. Then I wouldn't have to buy Surge out of my own pocket.
I agree with Brook. They're a great add-on item though.
If you're looking for something, I'd take a look at a fitness center, especially in small towns. My gym is located in a town of only 3500 people, but our membership averages around 200-250 per month (I also have tanning). I've been talking to Snap fitness the last couple months about opening a Snap (mine is independent) in a nearby town. According to the Snap guy, the gyms that are doing the best for Snap are located in small rural communities. I'd never open a Snap due to price, but I'd open another gym if I had the time. Used commercial gym equipment is extremely cheap right now.
If I worked 40-50 hours per week at my gym, I'd easily make $50-60k per year. However, I'm a pharmacist so it's not in my best interest to work that much there.