You hold down the Right toggle, on a PS4, but check for the platform you are on, to lock and unlock them. If you want to sell them they have to be unlocked, locking prevents accidentally selling something valuable when the menu skips about as you sell stuff and it rearranges the list, so if you are not checking every sale you can accidentally find a high end weapon or armour piece has made it to the top and if you aren't careful and its unlocked it is gone.
To dismantle, again on a PS4, hold down the left toggle and it will make a sound and show a graphic that when it goes fully around the item will be dismantled, check for your platform, it will be a similar mechanic just need to find the right buttons / controls.