Cross-realm trading is coming in Patch 10.1.5. The feature allows players to trade items and gold directly between characters from other realms in the same region.

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CROSS-REALM TRADING

Players will now be able to trade items and gold directly to characters on other realms in their region. Previously this was limited to things like conjured items or eligible raid/dungeon loot, but all those cross-realm restrictions will now be lifted. Play with your friends and trade whatever you want to whoever you want!