must done in-game. Moreover, in multiplayer, you RL Items can't search for individuals to exchange with outside of gatherings, nor is there a particular public space for that reason.
Rocket League exchanging: How to send a welcome。
Assuming you need to exchange with somebody, they should be in your Friends list. Then, at that point, either join their party or welcome them to yours, click www.lolga.com on their symbol (found in the lower bar along the edge of your profile) and select 'welcome to exchange'.