He isn’t reselling the assets as individual products, they’re just included in the game. Here’s a quote from the FAQ:
What can I do with content I obtain from the Marketplace and Learn tab?
Besides using this content for learning, experimenting, and prototyping, you can also ship it in your own products too! However, you can’t sell or sublicense Marketplace content to other developers for use in their products, e.g. via web site or e-commerce mechanism built into a 3D development tool.