It is really a matter of preference. Mixes are much easier. Scratch baking is a little trickier. You may want to experiment with different recipe so that you can perfect them and feel confident in baking them when ready to sell. Some people prefer to make doctored cakes which start with a mix. I prefer to bake from scratch but I do have many doctored cake recipes also. In case you are interested, here is a link for doctored cake recipes: https://docs.google.com/document/pub?id=1wBMz4wGYM-f4XKECyQEodFVwB0QHjQ2xQCeMG20bmws
If you want to bake professionally I strongly suggest baking from scratch. Clients can tell the difference. I have a certain strategy that I use every time, and when you pair that with good planning amd organization it becomes just as easy as using a mix.
I don’t like boxed stuff at all, taste is totally different than the ones you make from scratch. I agree with Goreti, try a few and stick to the ones you are comfortable with but I would avoid boxed stuff :-)
Scratch baking is my preferred way to bake for clients and family. I don’t want to have to rely on a company for my baking. If they change their recipe, box size, or discontinue a product then I’m starting over with my recipes. I’m also not a fan of the extra preservatives, and artificial extras added to their products. I also feel scratch has a much nicer flavor and texture.