As with used cars, 1-3 year old Thinkpads seem to be a pretty good sweet spot. New enough to be reasonably powerful / efficient / light, but old enough to be cheap and accessible.
Corporate IT shops are often selling off batches of laptops that they've replaced, so selection is usually good. The repair ecosystem will have been running for a while, so it'll be easy to get model specific upgrade and replacement parts on eBay for ~1/3 of what Lenovo charges (hinges, LCD panels, keyboards, trackpads, batteries, outer shell, motherboards, etc.).
You can usually find anything from "this laptop sat on a shelf for two years and looks pristine," to "previous owner was a slob, please take this off our hands." So depending on your comfort level and interest, you can either start using it right away, or buy every possible replacement/upgrade part and essentially build yourself a new custom machine.
Right now I'm using a T480 purchased new from Lenovo (for work), and another T480 that I bought on eBay for ~$600 and completely refurbished. I doubt I'll be buying a new personal laptop again -- rebuilding an old one is too much fun.