You can sell it for 50 shells and if didn't do the Sammy quest u can use the shell for buy the potion or if u did the Sammy quest u can save it for governor quest!
I haven’t done it in a while so I forgot how exactly but you have to look around the scene as Naira, Im pretty sure, trying to find a pile of logs and a piece of rope. You need to talk to the guy running the weapons shop to help you move the logs and build the bridge. Once ur Lina walk across the bridge and open it.