I’m making an Online Game. And I need a medieval buildings (like Castles), textures, packs, items in photorealistic quality. Can someone make it for me or work for me? I’m going to buy it.
For more question write it from Facebook: Redirecting...
First of all, how much are you willing to pay?
second, all the images on that facebook page are just regular marketplace content and their respected images.
Yes. I know that.
I’m going to contact with them.
I’m going to find sponsors.
After that i can pay. 
If you are unable to find what you are looking for on the marketplace as of the moment, you could always hire someone to create them for you. This will, of course, cost a considerable amount - depending on the complexity, quality and how many individual assets you require for your project.
There’s a place on the Unreal Engine forums where you can hire artists. It can be found here: https://forums.unrealengine.com/forumdisplay.php?25-Got-Skills-Looking-for-Talent
You can also request marketplace content on the forums (if there’s enough interest, it is possible someone will create it). This can be found here: https://forums.unrealengine.com/forumdisplay.php?56-Marketplace