There is a large marketplace for trading Rocket League items for both in-game currencies and cash. This was first deployed in 2016, but it has since undergone substantial modifications, such as the Trade-In system and other security features. The move from a normal loot system based on chests and keys to a much safer system based on blueprints and credits has been one of the most major adjustments.
https://www.lolga.com/rocket-league/trading-prices