My suggestion is similar to /mail. With /mailbox people could send another players items privately without needing for them to be online. The items could keep in some sort of vault like "buying items" in /ah. The commands could be simple.
/mailbox send - opens a mailbox inventory so that player can put oitems in there or take back if they were mistaken
/mailbox retrieve - opens a mailbox inventory where player can take the items they've recieved
/mailbox clear - clears all items from the mailbox
Bonus: maybe there could be /mailbox spam where you could set specific items to delete from mailbox automatically
