Cook potatoes in large pot of boiling salted water until just tender, about 10 minutes. Drain; transfer to large bowl. Drizzle pickle juices over potatoes and toss gently. Cool to room temperature.
Whisk mayonnaise, buttermilk, mustard, sugar, and pepper in medium bowl to blend. Pour over potatoes. Add eggs, onion, celery, and pickles and toss gently to blend. Season to taste with salt. (Can be made 8 hours ahead. Chill. Bring to room temperature before serving.)