this is similar to what happened to me. When I got it back it was locked to someone else's iCloud account and I had to call apple to unlock it.Had this same issue. I swear I disabled 'find my iPhone' and got an e-mail from Gazelle that it has activation lock. Said they use https://www.icloud.com/activationlock/ to find out.
Not sure what's going on but clearly it does not show up on www.cloud.com in regarding to this device. I e-mailed them to send the phone back to me as I will try somewhere else.
That's awesome that they did this. Their customer service is one of the reasons why I keep selling my phones to Gazelle.Update: E-mailed Gazelle Customer support and I told them that how it is possible that activation lock is in place when I clearly disabled find my iPhone and then erase all content and setting before sending it off. So I rejected the offer, got my phone back, only to realize that the activation lock e-mail wasn't mine.
I ended up e-mailing Gazelle Customer support that there is a mistake and that I don't recognize the e-mail. Even though the phone was mine, serial number and the imei # matched the box. Anyways, customer support was helpful and told me to send the phone back to them and that they would honor the original value that I locked into.
Got an e-mail notifying me that they have sent the check full amount.