Model comparison
Differences Between Mid-2014 and Mid-2015 Retina MacBook Pro: EveryMac.com
https://everymac.com/systems/apple/macbook_pro/macbook-pro-retina-display-faq/differences-between-macbook-pro-retina-display-mid-2014-mid-2015.htmlTop 10 Best Used and Refurbished Macbook in 2019 Reviews - TopBestSpec
https://topbestspec.com/best-used-refurbished-macbook/
General
Tips and checks for buying a used MacBook
https://www.cnet.com/how-to/tips-and-checks-for-buying-a-used-macbook/Buying a Used MacBook? Essential Tips to Consider - AppleToolBox
https://appletoolbox.com/buying-used-macbook-tips/5 Things to check before buying second hand Macbook
https://medium.com/the-brave-new-world/5-things-to-check-before-buying-second-hand-macbook-ac9e52c2de0612 Things to Check Before Buying Second-hand MacBook
https://mackeeper.com/blog/post/477-12-things-to-check-before-buying-second-hand-macbook/