dealshtc one®$100 trade in offer for $100.00

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This is directly from HTC and it's in addition to rebate money from the carriers.

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Actually, it's not from HTC. Read the Terms and Conditions. It's from a third party, Clover Wireless.
http://www.htctradeup.com/htcone/?q=terms_of_use
Clover Wireless is buying back your old phone, not HTC.