Rocket League in-game items, DLC content, and any progress, titles, or levels, are linked to a single account on a single platform. It is not possible to transfer any of these between accounts on different platforms. For example, if you have both a PlayStation account and an Xbox account, it is not possible to transfer or trade items between these accounts.
Items can only be traded to other accounts using the same platform. For example, Steam players can trade with other Steam players. Unfortunately, it is not possible to trade across different platforms.
If you have two copies of Rocket League on the same platform (2 Steam accounts for example), you can trade only the tradable items between the accounts. It is not possible to trade locked items, progress, titles, DLC, and other content even if you own both accounts.