The compass makes the work of every bounty hunter easier. It belongs to standard equipment of every robber. The look of the compass isn't changed by the Crime-Texturepack.
Normally the compass shows you the server-spawn. But if there is a user who got bounty, the compass shows you this user. So it makes it easier to find him. Alternatively you can see a robber with bounty as a red spot on your Map and with the command /c bounty.
You can buy a compass for just 1$ at the Snack-Shop. For the "buying-command" you have to use /shop buy compass or otherwise the item-ID 345. You do not have the opportunity to sell a compass.
Price: 1$ / ID: 345