Could you tell me, can I buy and download package n Unreal Marketplace and import that content to Unity 5? I’m thinking about 3d enviro models (Classic Room in Environments - UE Marketplace), so I supposed that this is possible. But how Epic see that problem? Is it legal? Can I do it?
Yes, you can.
However, there is an exception. All content provided by Epic cannot be used in any other engine.