Explain to the sellers that you REALLY need to move your plants into their new home before you move in. That should work.
The really good thing is that you are only moving two blocks away. I moved all of mine 12 hours away, and kept them out of the ground for about 2 weeks, and not potted up. I just left all the dirt on them, and stuck them in plastic bags. When the garden here was ready, I stuck the clump in the ground, watered them in, and they never missed a beat. The roots really weren't disturbed much, since I left all of the dirt on them, so they did just fine. I would think that by the time you are ready to start digging, that seed pod could be ready, so I wouldn't worry about that, either.
Good luck, and congratulations on your new home!
