Don't Vendor My Sets

Don't Vendor My Sets is a small addon that gives you a popup warning and plays a sound when you manually vendor an item that belongs to one of your equipment sets. The popup will ask if you meant to sell the item and mention which sets it belongs to - if you hit no on the popup, it will buyback the item.

It doesn't prevent addons from selling items or anything - it just tells you that you did sell the item (if you sold it manually) and asks if you were sure you wanted to do that.