You have to know this: There are three models of licensing for Epic Games:
- When is the free license, you are subject to UE EULA
- If you get the Basic license you get custom enterprise B2B agreement
- If you get Premium license you get Basic License + Unreal Developer Network (UDN) for support.
So, answering you question, if you create Blueprints, Meshes, Materiales, Archviz, etc from scratch, they are yours. You can sell them, distribute them, or keep them. If someone else buy your market content, he can use the content to make games, or whatever he wants but, not re-sell your content.
You neither are able in free license to take the core code from the engine and use it or modify it.
You can enter to UE Marketplace in order to take a look to the license model of the asset it contains.