If selling you'll need to understand the tax law and file (fed excise tax), not sure if that affects repairs, probably not.
You'd better be a LOT faster than I am if you expect to make a profit. After you build a couple rods and have recorded your time, figure that your time when you get good will be much less, but still may make your rods hard to sell while still making a decent rate for your time.
I suggest you simply try a few rods for yourself, starting with moderately priced blanks and components, then decide if you really want to go on. If you have a fishing family, there will be "demands."
Keep in mind that if you simply put together a lot of standard components, your rods will not be that distinctive compared to factory rods. If you custom make grips, butt knobs, do fancy thread wraps, you will have more time in the rods. Custom dimensions are faster, but still take time to make the decisions and plan the rods.
I think those who actually make a decent profit in rodbuilding are very skilled, very fast, and very good business people. You will need to consider that when you sell a rod you are going to be expected to stand behind the rod, and many people don't know how to properly treat a rod. You WILL get failures.
It may be in your circle that repairs may be more lucrative than building. Unless your tourney friends get their rods at deep discounts or free.
Big decision, sneak up on it.