Craigslist or eBay will get you the most return. Gazelle is decent for iPhones, but their offers for Macs are laughable and borderline robbery.
I sold my last 2 Macs on Craigslist for a little more than 50% of what I paid for them ~2 years prior. If you have a late 2013, you should be able to get 75% or more...but it might take some time. Just be sure to meet in a public place and accept cash only obviously. A coffee shop or mall food court works best. Good luck.