Amazon offers up to a $500 Amazon Gift Card when you trade-in a used, fully-functional Apple iPhone for AT&T, Verizon Wireless, or Sprint, as listed below. (Click the "Trade in here" button on the right side of the product page.) They're the best trade-in values we've seen on these iPhone models from Amazon. Examples:

• Apple iPhone 4S 64GB: Up to $500 credit
• Apple iPhone 4S 32GB: Up to $455 credit
• Apple iPhone 4S 16GB: Up to $415 credit
• Apple iPhone 4 32GB: Up to $300 credit
• Apple iPhone 4 16GB: Up to $290 credit
• Apple iPhone 3GS 16GB: Up to $155 credit
• Apple iPhone 3G 8GB: Up to $90 credit
• see more Apple iPhone trade-in deals at Amazon, with more carrier and color choices available