Places like wirefly, letstalk and amazon always offer on contract prices cheaper than the carriers. They have agreements where they get commissions for customers that sign up through them. So I would assume that those commissions are worth selling the device for cheaper. Also they have additional terms with getting the device. For instance, you have to keep data active on that line for 180 days or they can charge you the full price. So they don't want people upgrading for the cheap price and then selling the phone and taking data off that line. They get paid from the carriers by us keeping data plans. I seen stories where some get charged and others didn't. Whereas with a carrier they couldnt care less after you have signed that contract.
Read their terms of discount and you can see some of the additional things they want you to agree to when going through them. For users that change devices often, I wouldn't recommend. But for those expecting to keep the device for a while, I would.
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