How does one add an item to the exception list so as not to be sold? The useful/junk tabs do not seem to do anything as there is no way to interact with them. Thanks.

In case you haven't found a solution yet (quote from the tutorial):

• What if you want to tell Scrap which items to sell or not? Simply Drag it from your bags into the Scrap button. Alternatively, you may set a Keybinding at the Game Menu options. Press it while hovering the item.

Hope this helps!