So, something relevent to this thread I discovered after a few days of playing with my potion/herbalism shop set up.
Potions (from spellcasting) have 0 value assigned by the game, so they sell for only like 20 to 30 simoleans, even for the high end potions.
@Elliandre, that answers your question regarding how the game handles selling something with a value of 0.
I found this very disturbing, because it's actually quite a bit of effort to get spellcasting levels up, learn all the potions, gather all the ingredients. The lack of actual value for them is just...wrong. I found a mod, by Kuttoe, which fixes this by giving all potions a value. Even with the mod though, a lot of the potions are too cheap for my tastes, so I had to edit the mod to make things more expensive.
Just thought I'd share all this with this thread, seeing as others have also indicated they want to set up a potion shop.
Herbalism doesn't suffer from this shortcoming. Some of those concoctions sell for 1000+. Serums (from the scientist career), though, also have this problem, I believe. I was thinking of adding those to the potion shop as well once the sim I have planned for the career in my current game grows up.