From the FAQ:
Will I be able to build, buy and sell assets on the Marketplace?
Yes, we are building out this functionality. Right now everything on Marketplace is free, and we will release more free content soon.
Eventually all developers will be able to share and sell content through Marketplace. You will be able to distribute your own template games, content examples, plug-ins and more. We’re excited about enabling everyone to make money by releasing assets, tools and code for UE4!