Unfortunately the inventory system is run in-game so you can't really see others' inventories. But if you wanna do some trading you can check out /r/RocketLeagueExchange or https://www.rocket-league.com/trading. Fair warning: you meet some weird characters trading.
@Larochepiano I've used the Dribble Challenge and Obstacle Course maps. I'd recommend downloading the separate texture files so everything looks right but they're fun and help you work on your ball/car control, respectively.