If you want to spend $300 on a new bike, why not spend that money on a better rear wheel? The rest of the bike is in good order, and the rear axle broke. Broken rear axles were commonplace before the advent of freehubs and cassettes, which widen the bearing support. You have a freewheel now, which is screwed onto the rear hub. The axle broke from hard riding or a manufacturing defect. The LBS should replace it under warrenty.