Used Thinkpads are probably your best option. They are easily repairable, very sturdy, have good keyboards, lots of ports, the orange nipple in case you want to try living without a mouse, the hardware is mostly compatible with free operating systems, and yes, they can be dirt cheap due to frequent selloffs from enterprises.
You can buy them with coreboot or libreboot from Vikings, Ministry of Freedom, libiquity or find cheaper options on ebay or your local equivalent.
If you want a dirt-cheap ARM-based Linux laptop, you can buy a Pinebook, or a Pinebook Pro from pine64.
If you want a cheap chromebook, you can also buy a primebook.
There's plenty of options.
If you want a 100% repairable laptop, you can buy one from Framework, and build it yourself. It is mostly compatible with free software, and soon they are going to switch to coreboot.
If you want a corebooted Linux laptop, you can buy one from System76, Purism, Novacustom or Starlabs.
If you want a regular laptop with Linux, you can buy one from:
or even Lenovo, Dell or HP with their "developer editions".
If you want a truly secure and almost fully opensource ARM laptop which supports 9front and Linux, you can buy a MNT Reform.