That is incredibly upsetting. However, please be aware that selling in-game items is not permitted. This is in our EULA and Terms of Service. 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. More specifically, Crates, Keys, and other in-game items may only be traded for other in-game items, Crates, or Keys to ensure a safer and healthier ecosystem. Consider this a heads-up in plain English that attempting to sell in-game items personally or through third-party websites is not allowed.