Say I buy an animation pack for $25 in the marketplace and use it in a game I sell for $5 on steam. After their cut and UE4’s cut of $0.25 what are the royalties needed to pay to the person who made the animation pack?
If I buy content on the marketplace and use it in a game I sell, what are the royalties I have to pay?
No royalties are due to Marketplace developers; your responsibility there is only for the purchase price of the item.
Do I have to put their names in the credits?
No, you are not required to list the marketplace developers in the credits of your game.