Also check your model.
If you have a phone with the CDMA/GSM (Sprint/Global) chip, then you can make more money.
http://www.everymac.com/systems/apple/iphone/iphone-faq/differences-between-iphone-6-models.htmlCrazy but if you buy a phone from AT&T you get a GSM chip only phone which is actually worth less than the phones bought from Apple through Apple's payment plan. They are sold at the same price but there's a difference in the LTE chip which makes one worth more than the other when you decide to sell.
CDMA is mainly an American signal band adopted by Verizon and Sprint. So CDMA only phones cant really roam outside the US.
GSM is a global signal adopted by AT&T and T-Mobile they can roam pretty much anywhere.
Both is better
GSM less but can be sold globally
CDMA very limited to a few markets i believe