Are you sure it was that cap you sold? Unique and quest items do not show up in the trade window. After an act change, unique items change names, and can be sold.
I think the whatever/book inventory tabs both being active at the same time is a glitch whenever the inventory is opened the first time after loading the game or coming back from the battlefields or something. Someone else reported this, so I just ignored it the couple times I've run into it.
