Why do I get an "Unknown error while communicating with Rocket League Servers" error when using the trade-in feature in Rocket League?

If you're receiving this error while trying to use the trade-in feature in Rocket League, you can try the following troubleshooting steps 

1. Make sure all your items meet the Trade-in requirements per What is the trade-in feature in Rocket League?
2. Restart the game.
3. Verify your game files.
4. If the issue isn't solved, check for connectivity issues by following the steps in this article: Rocket League Connectivity Issues