I have successfully used Gazelle.com in the past to sell two Apple devices. I received the quoted amount and was satisfied. However, recently I sent them an iPad Pro 256GB (perfect condition) and they incorrectly evaluated it as an iPad Pro 32GB (with scratches and dents too) and wanted to lower my quoted amount by $105. I was not pleased with the new amount and called customer service. Gazelle, to their credit, did resolve this issue by giving me the original quoted amount after a series of phone calls and an agreement to re-evaluate my iPad Pro.
Even though my situation was successfully resolved, the whole time I was negotiating with Gazelle about my iPad Pro, I wished I'd taken pictures of it to document the condition and serial number. If needed, this would have made it much easier for me to prove what exactly I sent in and its condition.
Here is some advice if you decide to sell to them:
1. Take pictures of the serial number of your device
2. Take pictures of the over-all device condition
3. Take pictures of the device packaging
4. If possible, video the box being taped up and delivered to mail drop.
All of this makes it easier to prove what you sent in to them and what condition it was in when you mailed or shipped it.