To donate multiple items into the Clan Armory:
If anyone can improve upon this method, please let me know.
Firstly this requires you have two open tabs. Have the first tab open on the page where the item you are buying is listed. The second tab should be on the Trade screen.
Buy the item, when it asked where you want it equipped, hit the Back button.
On the Trade tab, hit Reload and donate to clan.
On the Buy tab, buy the item again and repeat the process.
(learned from CoZ)