As a primary buyer/user, my preference would be open source and, should an author choose to make their full source part of the Marketplace package, it would be a +1 from me in terms of buying it. Much like Unity and say, Ultimate FPS or ProBuilder. But if making full source a requirement prevented certain authors from working with Unreal Engine 4 due to “license restrictions”, and it almost certainly would, then I’d be in favor of allowing closed source as well.
I really enjoy working with Blender, but it is my understanding that their GPL license really torpedoes a lot of non-Blender compatibility. (Sorry, I’m really in over my head here - may not be 100% on target) It seems to me there’s a fair amount of people, especially in regards to the Blender Game Engine, who see the license and say “No thanks, we can’t do that.”
I don’t want UE4 to miss out because of license issues. All that said, I do think the argument that “UE4 is open source, so the marketplace content should be as well” has merit too. I’d probably support the idea of a clearly labelled ‘closed source’ / ‘open source’ on the description, and would probably buy from both. Forcing the user to go directly to the author’s website cuts Epic out of the revenue loop, removes some of the protections that Epic and their license offers, and potentially forces me to jump through hoops to keep everything managed rather than being able to do so through the marketplace.
Not to say I’d never buy directly - I did with Unity plugins a few times - but my preference would be to use the marketplace for the majority of purchases.