I have an art studio and rent out some of it to other artists. I pay the landlord directly, and collect $ from them. How should I list the $ they pay me? Uncommon income?

It is income on the Sch C no matter which income section you put it in. 

If your art studio is a SCH C business, then all income received by the business from any source for any reason is business income reported on the SCH C. Period. Then the rent you pay the landlord is a SCH C expense.