I’m not sure if the assets in the Marketplace are royalty free or not. it’s not specified or maybe I missed the indications.
They all are. Marketplace publishers can’t charge any roys.
They mostly got the same license, royalty free and for use in any “packaged” product.
Some of the free Epic stuff you are only allowed to use in UE based products.
You could use marketplace content in your own products (source: https://www.unrealengine.com/faq search for “What can I do with content I obtain from the Marketplace and Learn tab?”).
If you buy something from the marketplace you would get the “lifetime right to use the content in developing an unlimited number of products and in shipping those products” (source: https://publish.unrealengine.com/faq search for “What can users do with my content once they purchase and download it?”). So in short: If you finish your game and use that content in some baked form then it’s ok to sell your game (and pay Epic 5%). But it’s not ok to buy a “red apple asset set” and sell it as “green apple asset set” to other developers.
It’s pretty much the same with the stuff from the learn tab (and free stuff from Epic games in the market place like the infinity blade assets: https://www.unrealengine.com/marketplace/infinity-blade-fire-lands) but in that case it’s “Licensed for Use Only with Unreal Engine-based Products”. It’s ok to use some assets from the learntab in your Unreal Engine game but it would not be ok to get the CAR character from the infiltrator demo (https://forums.unrealengine.com/showthread.php?84608-Infiltrator-Released-For-Free!&highlight=infiltrator+demo) or the warriors of the Inifinity Blade assets and create a game in Unity 5 or Cryengine with it for example. Good reason to choose UE4
So in my opinion the marketplace and learntab stuff is pretty cool if you need some “common” assets that does not have to be exclusive in your game because that could speed up your development a lot. It does not matter that much if a common rock looks similar to a rock in another game or if a AK47 looks similar to another AK47. You could create it crappy or polished but this rifle just looks like it looks and everybody knows it. But it is not the right place to get your very own version of a unique “Super Mario” character or to get your personal unique “Star Wars” intro music because everybody else in the world may buy and use market place or learn tab assets as well.