Psyonix constantly releases free challenges. Complete enough weekly challenges and season events (like Haunted Hallows or Frosty Fest) to earn limited-edition items that cannot be obtained anywhere else. These event items are often just as cool as Black Market decals.
To safely and permanently add items to your collection, use the official methods supported by Epic Games
If you are looking to expand your inventory safely, consider these official methods:
Exploring "unlock all" tools for Rocket League often leads to a mix of legitimate visual mods and risky third-party software. While some community tools allow you to change your car's appearance locally, any program claiming to permanently unlock paid or rare items for use in online matches is likely a scam or a violation of the Rocket League Code of Conduct Legitimate Customization Tools