I’m not a lawyer, this is just how I interpret the rules for my own use.
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Anything I buy or get for free in the marketplace, and anything I build with the UE, I can sell for however much or little I want. Once I hit a certain revenue threshold, I need to start sending royalties to EPIC for the Unreal Engine use.
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Same thing with EPIC assets in the marketplace. The main difference is that EPIC assets cannot ever be used OUTSIDE unreal engine; some marketplace sellers (but not all of them) allow you to use the things you purchased, outside unreal engine.
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Whatever assets you get from elsewhere (either you make them yourself, or you get them through legitimate means) are OK to build and sell. This is dependent on the license you have to the assets you got elsewhere. Make sure that license allows game distribution. EPIC doesn’t care about other people’s licenses one way or another (except they might care that things you sell in the EPIC store are correctly licensed!)
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Each asset may have a different requirement for credit or not. Assets from the store, and from EPIC, generally do NOT have specific credit requirements, although the Unreal Engine itself may have such requirements. Also, you generally need to make sure that your own copyright is protected with the proper notice – once that happens, the licensed content you use generally ends up under the same protection.