My suggestion would be to get a 3D Printer and print a few simple things. See if you like it. See if the amount of time it takes to print can produce a viable business model. The fact is the learning curve is so long and steep, you probably don't want to approach the topic this way. I realize that isn't the answer you are looking for, but if you don't really enjoy solving these kinds of problems that show up when printing, when using a new filament roll, when using a new color, when slicing at a different layer thickness, etc, etc, etc... You are going to hate it!

And that doesn't even address the fact that until you have lots of experience designing and printing parts, you don't even know what can go wrong. If somebody says "I need a part that looks like this." A lot of thinking has to happen to turn that into a design that can even be printed. There are issues like support material that take a long time to learn. There are issues like how thick (or thin) of a shell can you safely get away with. There are issues about which type of plastic is most suitable for a given application.