I think I am getting somewhere
Sheamus - Pehaps you're conflating at least two things here which perhaps contribute to a muddle.
If you set up an art gallery and if artists consent to exhibit/sell their work through your gallery they are not "amateurs" they are professionals selling their work; they are paying tax on their income (an artist is a one-wo(man) business) less the cost of materials, studio rental, time making the stuff etc.
Your income from a gallery exhibiting and selling work of living artists IS "the commission". In my experience as an exhibiting artist I (the artist) sets the price s(he) wants for the artwork. That is the figure which goes in the catalogue. The gallery then deducts a pre-agreed percentage.
The artist records the sale-price minus the gallery percentage as taxable income.
I understood the system to be that the gallery adds up all the "percentages" from sales of different artists' works and that (less publicity, cost of private viewings, premises, employment of staff etc. plus high insurance of unique works on your premises!!!) is the gallery's "income" and is taxable.
Without wishing to put you off your project, attracting artists to exhibit through a particular gallery, and attracting the right buyers, is highly skilled disciplined stuff and requires contacts, business acumen and in-depth knowledge of art history as well as understanding of contemporary art.