Does anyone know of any sort of open source or ecommerce software code that would allow me to host a site where people can sell stuff and it would give me a commission for facilitating the sales?

Do I need ecommerce? This is important. Generally speaking, consumers are comfortable making purchases online for under $250. If your product or service costs more than that, then you may be better served creating a "Lead Generation" web site and an eSales team who are comfortable working with internet based leads.

Are you looking for more of auction based functionality than straight e-commerce? Once users start posting products for sale, then you start to move more towards this model.

I was able to do something near this with AdsManager component (Joomla!). I made people pay for annoncing X products.

If you want Open Source then you're either going to want to go with Joomla or OsCommerce.