Starting an art shop can be super stressful.
But honestly, I think as long as you have your rules; aka what people can request, what you won't do, how to credit, etc.
Have some examples for them to see
and then some basic pricing
headshot - just the head of character
bust Shot - Down to the shoulders
half body - half of the character (usually down to the waist)
fullbody - the whole character
Then you can decide if you want to sell different things.
sketches(not clean lines), lined(clean lines no color), flat colors (aka no shading), shaded, etc.
Pricing wise, people are super helpful if you post examples and ask; what would y'all pay?
I hope this helps! :3