Some good answers here.

I would add that network effects can be a curse if you aren't careful:

E.g. say you can’t get sellers to pay for permanent placement on a listing app like yours, if they leave the app is less valuable to buyers, so then buyers stop coming. Less buyers means the app is less valuable to the sellers. It's a vicious cycle.

People using something lots doesn’t mean they’ll pay for it - whichever route you go, do as much research as possible so that when you open up the revenue taps, people will pay to use your service. People need to feel they can't live without it after using your service. Sell painkillers, not candy.