It might not be on the roadmap because it’s not directly related to the engine itself, perhaps because they want to keep the goods secret until they think its ready?
They have said in the past that the marketplace is one of their top priorities.
Making a fully fledged shop like this is not a peace of cake. The database is complex, the interfaces need to feel just right both on the frontend and backend, the payment system need to be secure and all legal stuff including the terms of use etc need to be written from scratch for all countries and on top of that the entire service need to work for thousands of concurrent users.
Either ways, this thing is a money machine and every month they don’t have it could be considered actual profit loss so don’t worry for them, I’m pretty sure they know what they have to do 