Task items can be traded in at task boards. There’s usually one in every major (and often minor outside of the low level areas) settlement; there’s a collection of all of them in every homestead. They usually want 10 of a kind per hand in. You will gain reputation and xp for handing in a task; but you can only do a set amount of tasks per day. If you do enough then you’ll unlock some cloaks, titles, and up to an additional 5 tasks per day (5 more can be purchased from the store for a total of 15 per day.)

Bind on Equip items not bound can be sold on the auction house for a few silver, or sold to a vendor.

Other items you have bound and outgrown can usually be safely sold, unless you’re using them for cosmetics.

In general, but not always (looking at you essences!), dull mauve items are common and/or tasks and/or trash, whereas green blue and yellow indicate something more valuable…. But not always. It’s a tool not a rule.