Do people actually buy used computers and sell them? Like, apart from graphics cards...

Seems odd to me. Most computers I've ever had last for ~10 years, at which point resale value is definitely zero...

Similar with phones and tablets: yes, people do that, but it's not the common case. I've only ever sold computing devices within the first month of ownership, after realising I made a mistake in purchasing it (not a broken device, but just not meeting my needs), but on second-hand sites you can also see various people offering them up after 1 or 2 years because they want something shinier and the old system still has value. I've bought my current phone from someone like that, and as a student I also bought such a laptop

I don't sell them but I buy exclusively used computers. All of them coming from the refurbishing market of business computers.

From a purely financial perspective buying a computer or vehicle brand new is a very bad idea.

yeah, I buy preowned laptops fairly often to be fair.