When Rocket League Items Shop the two players have added their exchange you bought Rocket League before the Rocket League Items allowed to-play update, you can overlook this limitation.
You can exchange with players on a similar stage as you, but since Rocket League has cross-movement, you can sign in to any stage to see your tradeable things. There are a few things you can't exchange, including concealed Blueprints, things bought from the shop, and extra presents....