Unity Asset Store - Unity's asset marketplace for game makers.
GameDev Market - GameDev Market is a marketplace for high quality, affordable 2D, 3D, GUI & Audio game assets, handcrafted by talented creators around the world.
Kenney Assets - We've created over 40,000 images, audio files and 3D models for you to use in your projects. Thanks to our public domain license you're even allowed to use them in commercial projects!
Sell My App - Sellmyapp is a top quality mobile source codes marketplace with strong community elements
OpenGameArt.org - A site dedicated to sharing artwork & other assets for game development.
UE Marketplace - Marketplace for game assets and tooling for Unreal Engine.