Fishrrman's rules for used MacBook buying:
DO NOT BUY:
MacBook (Retina, 12-inch, Early 2015)
MacBook (Retina, 12-inch, Early 2016)
MacBook (Retina, 12-inch, 2017)
MacBook Air (Retina, 13-inch, 2018)
MacBook Air (Retina, 13-inch, 2019)
MacBook Pro (13-inch, 2016, Two Thunderbolt 3 Ports)
MacBook Pro (13-inch, 2017, Two Thunderbolt 3 Ports)
MacBook Pro (13-inch, 2019, Two Thunderbolt 3 ports)
MacBook Pro (13-inch, 2016, Four Thunderbolt 3 Ports)
MacBook Pro (13-inch, 2017, Four Thunderbolt 3 Ports)
MacBook Pro (15-inch, 2016)
MacBook Pro (15-inch, 2017)
MacBook Pro (13-inch, 2018, Four Thunderbolt 3 Ports)
MacBook Pro (15-inch, 2018)
MacBook Pro (13-inch, 2019, Four Thunderbolt 3 Ports)
MacBook Pro (15-inch, 2019)
ALL of these have the disastrous "butterfly keyboards" that are highly-prone to failure. Although Apple has a free replacement program running for 4 years "from new", when that time expires YOU will pay for the repair.
And it's NOT CHEAP -- $750 for even a single key gone bad.
That's because the entire top case has to be replaced... even for a single key failure!
DO BUY:
MacBook Pro 13" -- 2020
MacBook Pro 16" -- 2019 and later.
These have the new "magic" (scissors) keyboards, as did the 2015 and earlier MBPs. These keyboards have been very reliable.
Also:
MacBook Pro 14" or 16" -- 2021