I have to agree with SE_JonF. Also if you ask me, the marketplace should be only about the engine and its tools, it’s a wrong move to include assets specific to a particular game in the marketplace, let it be UT, fortnight, GameXYZ, doesn’t make a difference.
There are two options for community members creating mods for Unreal Tournament and wanting to use content created by others.
Any music or other assets purchased in the Unreal Engine Marketplace can be used in any game or mods for any game, including Unreal Tournament. The terms are described on https://www.unrealengine.com/marketplace/marketplace-business-terms-faq, and equate to a one-time price for unlimited royalty-free use by an individual or company and its employees or contractors.
Any non-Marketplace content created by others can be included in games if there is a mutual agreement between the buyer and the seller. The compensation can take any form, such as royalties, a one-time fee, recurring payments, etc.