In a large saucepan, cover potatoes with 2 inches of water; season with salt. Bring to a boil; cook until easily pierced with a knife, 12 to 15 minutes. Drain and let stand until cool enough to handle, about 15 minutes.

Meanwhile, in a large bowl, whisk together vinegar, agave, 2 teaspoons salt, 1/4 teaspoon pepper, celery seeds, and dill. Add celery and onion. Quarter potatoes and add to bowl along with oil; toss to coat. Serve, or refrigerate in an airtight container up to 1 day.