In Borderlands 3 players can trade with each other at any time and can do so by using a variety of methods. Players can send each other mail through the in-game social options and mailbox system. Or they can trade when playing in the same game by using the trading features and accepting trades.
Walk up to another player and hold either the X Button or Square Button
When you are playing with other players in Borderlands 3 you can simply walk up to them and you will see menu options appear on their character. You have the option to trade or view their profile. To initiate a trade you will need to press and hold the X Button (Xbox) or Square Button (PS4).
Assuming the other player accepts, you will then be taken to another menu which allows you to see which items you are giving and which you are receiving. You will have the option to trade or duel for the items. Trading does have to have both parties accepting to the items in the trade and to trading itself, otherwise it will just send them an invitation. You DO NOT have to be friends to trade in Borderlands 3.
You can also gift items to players without trading
Another method of “trading” which is really more of gifting is by using the mailbox to gift items to other players by sending them mail. Unlike trading you do need to be friends with another player to gift them items through the mail system.
Trading in Borderlands does allow players to divvy up the loot fairly when playing co-op and maximize any weapon mods or trinkets to make the team more powerful overall