Are you on the Next 12 plan? When I did the comparison, the amount AT&T is "forgiving me" by turning in my 6 Plus with 128GB was $10 more than Gazelle's offer. That's likely because the premium I paid for the Plus version and the extra memory is never recouped when resold. At least AT&T gives every phone the same percentage of the original cost when they give you credit for turning it in. This just proves every situation is different.
I've used Gazelle before and have a few suggestions. Ask AT&T to unlock your 6 so you can get the higher price from Gazelle for it. Last year they made me wait two week ostensibly so I couldn't return the new phone I was purchasing and get the unlock unjustly. After the two weeks is over, AT&T will unlock it and you'll have to restore it from iTunes to complete, so you'll need to wait before mailing no it off. Also, from my experience, Gazelle never gives you the Best condition price no mater how nice you think it is. Unless it's just been taken out of the box, expect to get the Good condition price.
Thanks. I think they will give me the price they promised. It is in great shape
Next year, I buy the phone and forget all this mumbo jumbo. Getting too old.
It is in the box and on the way. I did take out the sim card, as per their request. Sure would like to know exactly what data that card contains