I know this thread is a few years old now, but it's one of the only things that comes up when you do a search engine search for GadgetGone. I decided to give them a shot-- and seeing as there's an abundance of skepticism in this thread due to reviews being given by new users, and being that my profile is the opposite of "new"-- I've been a Macrumor's forum user for just over 15 years-- I thought I'd weigh in for anyone that might be considering this company to sell their devices. (Of course I have the benefit of this company no longer being *brand new*, they've been around for a few years now and their online experience is no doubt dramatically better than it was when they first launched, based on what I read here).
Obviously I was primarily interested in GadgetGone for a simple way to offload some gear and because they had notably higher offers / quotes than other, older trade in companies. Apple was not accepting trade-ins in store locally, and I could see no way to trade in 2 devices at once with Apple otherwise, so I looked elsewhere and actually ended up getting more money this way.
So how was it? I am more than satisfied. Process was quick to get an offer, all I had to do was print the prepaid shipping label and send it off. They evaluated my iPad Pros and paid me the day after the items were delivered by USPS (sent them a 2015 12.9" and a 2016 9.7" with my first round.) I did choose to get paid via PayPal so I of course got dinged the usual 2.9% by Paypal, and they let you know that up front when you choose that payment method. I wasn't interested in an Amazon gift card or waiting for a paper check to be mailed.
Happy with my first experience, I sold them a 2018 iPad Pro as well a week or so later. Again, significantly higher offer than any other place (by more than $100 anyways). Same experience-- everything was smooth, they evaluated my iPad and paid me the original offer amount the day following delivery by USPS.
Thumbs up. I'd use them again.