I spend about 2-3 hour per week running my business of buying and then reselling clothing online. I have sold over 100 items in the past year and plan to expand more. It has taught me many things about business/communication/marketing and I consider it to be much more than just a way to make money. Could I list this as an extracurricular on my application?
Yes.
Yes, especially if you apply to business schools.
Absolutely. You may want to keep a journal of sorts so you can look back on what you discovered/learned so later you can write an essay