I've bought second user laptops, 3 or 4 over the years. I wouldn't buy any of the other stuff you mention, on the basis that the only reason it is on resale is that it's faulty or worn out.
Why are laptops different? As the title says, it depends what you buy and from whom. I buy ex-lease or refurbished business class machines, from reputable sellers, such as retail stores and auction houses. I don't buy from individuals on eBay! I also stick to well known brands, such as Lenovo and HP. Dell is probably OK too but they have a reputation for using "not quite standard" parts, for example the PSU on some older desktops.
I've just retired an IBM Thinkpad T23 around 16 years old. It's become a bit hard to start - I think I know what the problem is but haven't had time to look at it yet.
I'm still using an IBM T43 from 2002 daily running Windows 7. and my latest acquisition about a year ago was an ex-lease Lenovo T500, about four years old, in pristine condition for thirty nine dollars Australian, about $60 US from an online auction house here in Australia. Came with a 30 day warranty. At that price, it came without an operating system but I installed OpenSuSE Linux and it found drivers for everything in the machine, including motherboard headers for optional features not installed. Runs like a dream.
As Bob says, there is a risk in buying second hand but if you buy a quality product from a source you trust, with a straightforward return policy (like the auction house) and providing you know what you are doing, I think the savings are worth the risk.
Your mileage may, of course, vary.