How do I sell old items?

In the first Infinity Blade, when you go to the Store you would see all items, and for those in your inventory that aren't equipped you had the option to sell them back. I found this very helpful to saving up for the more expensive items available late in the game.

However, in Infinity Blade 2 the Store section only shows items that aren't in your inventory. When I look at a non-equipped item in my inventory, I see a button with the money symbol on it with a value next to it, however it is always disabled.

So how do I sell old items that I've mastered and no longer want?

It appears that in the new game, you can only sell items you have duplicates of. This tweet appears to be from an Epic exec confirming this.

I think that's crazy annoying, but no one asked.

I actually just posted a related question, as that struck me as odd and undesirable.