The lock is the most important tool to protect your chests. It prevents that other robbers can just open your chest. The more locks are on a chest, the more crowbars are necessary to break it open. The maximum amount of locks on a chest is 9. You can see how many locks are on a chest by using a nametag. The first lock is included with placing a chest. You can set a password by using /c setpw *Password*. The texture of this item is changed by the Crime Texturepack and so it really looks like a golden lock.
After purchase you just have to right-click on a chest with a lock in your hand to reinforce the chest with another lock. These locks disappear when the chest is broken and replaced.
Locks can be bought at the Technician in the TV-Tower for 170$. There is no possibility to resell it to a shop. Of course you can still try to sell it to another robber. The item-ID which has to be entered at purchase is 369, alternatively the item-Name blaze_rod can also be used.
Price: 170$ / ID: 369