By the time said macbook is fully supported, it is in sorry state. Judging by what I found when looking for second hand laptops, macbooks don't age well.
The problem with used Macs is that they're still really expensive, and if you want to only run them with Apple software, the support duration is limited (especially true for phones, even more for Android ones). You can pick up used Thinkpads or Dells in great condition for very cheap after a few years.
Are you dumpster diving for your MacBooks, perchance? I buy all of mine second hand, they are all still mint by the 5 year mark, and I have several at the 10 year mark that all I've had to replace is the battery.