Psyonix says you can disconnect platforms and switch primarys if you change your mind, but you will not be able to link any platforms you remove to another Epic Games account. When it comes to items in your inventory, the feature will allow you to share all the points earned for free, season bonuses, in-store purchases, Rocket Pass items, projects and Rocket League-Lodified DLC. However, you cannot share the exclusive items and credits of the platform. You may not even share Premium DLC Packages, but Psyonix is working to make them accessible across all platforms in the future.
Finally, the developer has clarified that you can only trade items purchased on the platform where you purchased them to reduce the risk of fraud. You can’t get on the Xbox to market something you bought on the PlayStation, for example. You will need at least 500 credits to trade, unless you have been playing Rocket League before playing the free start.