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Apple does discontinue the sale of the old models when they introduce the new ones... so there are no old models to buy when the new ones are in the stores.
Unless you happen to live in Japan. They have quite a few 'Used Apple' stores here where you can save 20 - 30% on old Apple goods. Surprised no one has opened stores like this in the US. I picked up a used Ipod touch the other day for about $100 US that works great.