With eBay I have to photograph it, write it up, list it, answer ridiculous questions about shipping it to the 3rd world and take payment in the form of casino chips mailed to me in a few months. Then I have to deal with the buyer, ship it, and the whole time I have to be worried about scammers. All the time I'm competing against companies that are listing 50 iPhones a day because that's their fulltime job and my account hasn't been used in 4 years. I wouldn't buy from me in that circumstance.
With Craigslist, things are a bit worse. Not only do you get occasional scammer emails, but I have to meet the person to physically sell the item. I've had people stand me up numerous times, be continually able to schedule, etc. And when I do meet up with someone, they could give me fake money, or just steal the item, or refuse to buy it meaning I have to go through the whole thing again.
With Gazelle I got a perfectly fair bid. I hit accept and in a day or two a box showed up on my door. Put the phone in it, dropped it off at a UPS store, and in a few days I got my Amazon gift card. No weird fraudsters, and I spent a total of about 10 minutes of my time. In return I got a price I was quite happy with, an easy experience, and I was able to use the money to help support my Amazon habit.
Worst case scenario, Gazelle could have said no and shipped the phone back to me. I wouldn't be out any money (unlike if I paid for eBay shipping), and I could have sold it on Craigslist.