Have some unused electronics lying around? Your first bet would be to sell on eBay or maybe Craigslist, but in my experience you will earn the most on Swappa.
Swappa.com allows users to buy/sell Apple or Samsung devices with the only fee being a $10 listing fee. Given that most phones are worth upwards of $100, this is much better than eBay's 10% fee. Plus you don't have to worry about that annoying 21-day hold for new sellers.
Swappa is easier to use than eBay, listings are simpler and fees are lower. It's also easy to sell and get a good price for your electronics even if you are a new seller. Comment if you have any questions.
I have been re-selling electronics for 5 years and I am glad that I started using Swappa a couple of years ago. Screw you eBay!