Linux

first off, "lenovo" is not the thing to get, it's just a subset of those - thinkpads. and even then, not all of those - just the T and P series. those are the "pro" lines, durable, dependable, expandable, serviceable, and widely used. so when corporations swap out their fleet for new models, they flood the market and hence can be had for cheap. multiple generations of the same model are cross-generation compatible, so they share the same peripherals, like docks, have interchangeable parts, like keyboards, displays, etc.

don't get used ideapads, thinkbooks, thinkpad E/L series, etc. those are either consumer-class models, have substandard features, are incompatible with each other, etc. don't get the yogas and S-suffix models, as you'll have a removed time servicing and/or upgrading those.

the whole point of getting something used, i.e. something that was touched and rubbed and spat all over, is if it's a) in good enough shape and b) you get it for cheap. you took care of of item A when going for thinkpad T-series and you're compromising on item B if you're going through an intermediary.

them dudes you mention are skinning you alive - 500 EUR for a T14 G1 is insane, it should be less than half of that. I also like how they're including none of the tech specs which just ups the ick factor.