The other thing that can happen is your delivery box can be full. When gil from a sale cannot enter a full delivery box, the game responds by graying out your sold or returned items on the ah, making them unclickable and in essence, the slots become unusable. To correct this, make sure you take some items out of the dbox to get things flowing again, wait a while for the game to recognize there is room (can take anywhere from hours to days), and then go back to the ah and see if your sold/returned items are clickable now so that they can be sent to your dbox.