If you agreed to participate in such a transaction, we cannot help you.
Remember: Outside trading puts you at risk of scams and other illegal activity. This is why trying to sell or purchase in-game items outside of Rocket League is against our EULA and TOS. Blueprints, Credits and other in-game items may only be traded in-game. This is to ensure a safer and healthier ecosystem. Attempting to sell in-game items personally or through third-party websites is not allowed.