A bicycle worth riding is worth keeping. The owner has a lock for the bike, but didn't think they needed to use it. Select High Security Locks and Cable Don't be fooled by a manufacturer's claim of a burglar proof lock. There is no lock that will withstand a forceful attack.
Common tools used by a bicycle thief are bolt or cable cutters.
Try not to buy a chain that is hardened all the way through. Sometimes a one hundred percent hardened chain can be broken with a hammer blow. A non hardened inner core will still make it difficult to defeat with a hammer or bolt cutters, but the hardened outer jacket protects the chain from hacksaws. If you decide to use a cable, select one at least six feet in length so the frame and front tire can be secured. Remember, the heavier the chain the better.