I've had two different experiences with Gazelle, one good and one bad.
The Good Experience -- I purchased a used iPhone 6, unlocked, that was listed as Excellent. The phone was pristine and haven't had any problems at all. Been using it for 3 years.
The Bad Experience -- Spouse purchased a used iPhone 6, unlocked, that was listed as either Good or Fair (can't remember). Within 3 months the battery inside expanded and pushed the screen off. When I took it to an Apple repair location they informed me that the screen as after market and, in order to do the repair, they would have to put on a genuine Apple screen for $349 dollars. No other repair place would touch it. We ended up doing the repair ourself with the same aftermarket screen.