It is one of the rarer ones but that's way too high! Just because the seller is asking that much doesn't mean it will be overpriced for 10 years though. It depends what someone actually buys it for. On the flipside I do believe that sellers asking prices affect the market a little, even with non-sales, but we as buyers should be firm in not being influenced by it.
That's a tough one. the buyer usually wants to make a deal as low as possible. The seller, as high as possible. If they are both firm, then there's no deal & no one is happy in that case. There's usually some flexibility on both ends, since both have an interest in making a deal. As more & more people start collecting postcard sets, I think the pressure to backfill & the limited supply will raise the earlier series prices up, especially for the nicer or harder to find sketches. I've always thought the sketch card revolution started with Smokin' Joe's 4th series postcard sketches, so IMO I think this is an important one and will command a much higher price. With all that being said, I think the $600 is high, but there are certainly some "wacky" collectors in this hobby, so all we can do is wait & see.