On the official Steam declaration page, Psyonix gives a helpful guide regarding Rocket League Items what's tradeable and what isn't. Here's that manage in full:

The designers behind Rocket League are getting ready to free the round of its plunder boxes, a framework for randomized in-game buys pundits express adds up to betting. While most clients hail the choice as an invite advancement, the change takes steps to overturn one of the game's most intensely contributed networks — its dark market.Currently, players can buy plunder boxes (known as containers in the game) to procure custom wheels, decals or vehicles to use in the game. There's no assurance what you'll get when you open another crate, each containing one thing from a bunch of likely contributions with different degrees of extraordinariness.

All that is evolving. Psyonix, the game's designer, and parent organization Epic Games declared recently that plunder containers will be supplanted with in-game buys where clients will know the "specific things you're purchasing ahead of time," eliminating the current component of karma. In any case, that haphazardness was a factor that profited a gathering of players who amassed in-game things and afterward either exchanged or offered them to players who liked to pay a premium than spend their cash on the unsure possibility of handling their ideal thing Buy Rocket League Credits in a plunder crate.