In the past year one of the most common questions I get is “My (insert name of item here) has been stolen and placed on craigslist. What do I do?” That answer is a very simple one. Call your local police and enlist their help.
I know there have been stories on here where citizens have taken matters into their own hands and have been successful in getting their stuff back but I do not recommend that. It only takes a split second for a situation like that to turn into a tragic one.
Please leave law enforcement in the hands of professionals.