Ive sold Macbooks on ebay before but never bought one. What is the oldest air you would buy that would still "current" late 2018? or should i stick with the 2019s?
If you concern with butterfly keyboard, try avoiding 2015-2019 macbook. If I were you, I would wait next year to get used MBA 2020 or get new one the basic model.
Personally I would only go with a 2020 - much better CPU/GPU performance, double the standard storage, and most importantly the latest generation keyboard. You can get them in the Apple refurb store or occasional sales from third party resellers