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2.8 Please note that you may not order more than one product of each product line per transaction. For the avoidance of doubt, please note that you may order several Products in a single transaction as long as these Products don’t belong to the same Product line. For instance, you may order an iPod shuffle and an iPod touch, but you may not order 2 iPod touch. 

Why am I allowed to purchase one product from each line only? Answer: Grey Market & Reselling.

(Source: store.apple.com)