I don’t want to speak for EPIC, but my guess (because it’s what I would do) is that they are “forcing” the marketplace to use Blueprints because that’s their new baby and it’s one way of making sure Blueprints can do everything they want them to. I’d imagine once Blueprint functionality == C++ functionality (or close) they’d consider opening the marketplace to everyone – UNLESS the idea behind having a Blueprint marketplace is to insure the buyer has full source code…