If there's an adjuster on the rear derailer, turn it counter clockwise one full turn and go out for a ride. Try out the shifting. If it gets better but doesn't go away, try another half-turn until it shifts correctly. If the previous owner only put 50 miles on it, your cable might be stretching. This is why most shops recommend bringing a bike back for that free 30 day tune-up, or 300-500 miles.