I don't have multisite, but I want to take a percentage of each sale

Hi, I want my members to be able to post products for sale and for me to take a percentage of that sale. I've seen a bunch of Q&A's here about Multisite and going into Settings/MarketPress to do that.

- I don't have multisite
- there is no choice of MarketPress in my Settings tab.

I'm not sure what to do here. Thanks!