We Asked for It, We (Finally) Got It

From the U24 release notes:

Players can now trade multiple items at a time when using a Barter UI. For example, players can now turn in multiple Treasure Map Pieces during the Crystal Cove event, or craft multiple items using the Cannith Crafting system. There is a new UI in the Barter UI that allows a player to enter how many times to trade the required ingredients. This trade can be stopped in-progress by pressing the "X" button located near the Amount UI.

I'm sure I speak for many of us:

'Bout bloody time.