Of course I had to look this up as I have never used Gazelle. As far as a percentage cut or any fees they take out, I am not positive as I can't find an exact answer. But I did find this.
Is there any cost or obligation to use Gazelle?
"There is no cost to having a Gazelle account or setting up a transaction with us. We pay for shipping, and if we send you a box but you decide to cancel your order, there is no charge! We would kindly ask that you recycle any unused shipping kits. You are under no obligation to send the item to us, and if you decide not to, there are never any penalties!
With an active account, you can keep track of all your transactions and also save time by logging your shipping and payment information for your next transaction. We will also keep you up to date on special Gazelle promotions such as give-aways, offer bonuses, updates on major technology releases, contests, and even the opportunity to create a watch list so you can be notified when an offer on one of your devices increases!
Gazelle is about building relationships for years to come as you cycle through device after device. You can always be sure you have a reliable and trustworthy place to bring your used devices."
I've used Gazelle several times to sell MacBooks and mobile devices. You generally get less than you would if selling via Craigslist or eBay, but still a very fair market price. Their service is excellent. Shipping to them is easy and free, they assess your device's condition quickly, and they pay you quickly.