There are risk to both suggestions:
1. Craigslist takes some time and effort, and does require some knowledge. (But gaining that knowledge will benefit you in keeping your bike maintained.)
2. Having the Mongoose repaired may end up just polishing a ....
(I picked up a Mongoose off the curb a couple of years ago. Fixed the brakes, stripped off the derailleur and shifters and made it a single-speed; it was OK for a beach beater but I lent it to my brother when his bike got stolen and he managed to break the crankset within 2 weeks.)

Like others have suggested - If you have a friend who knows anything about bikes, have them take a look at it and see if it can be adjusted, lubed. and cleaned. But think twice before spending much money on it.