Is there any way to show only free items ?
No, there are no filters on the marketplace.
Most free assets are from Epic Games:
Few other ones I remember:
You could find some high-quality assets in the learn-tab of the UE4-launcher as well.
For free assets check my thread: Free Community Assets - Community Content, Tools and Tutorials - Unreal Engine Forums
Marketplace its focused on paid, even if you want to publish something there is a minimum price, only epic games can (with some exception):