People are always surprised to find this out. I see people taking the CPU out of an old computer -- one of the most reliable components -- thinking that it's somehow worth something after throwing out the motherboard that it works with.
Now working a 90s computer with an ISA slot in it will sell for over $100 online. They have become rare. Be sure to not power one up if the on board battery has leaked until it is properly cleaned. It may be necessary to use acid to neutralize it and then wash it off with water, dry it, and desolder the battery completly. ISA slot computers are needed for industrial control and other specialized boards that cannot work an any newer non ISA system! Any working 5+1/4 floppy drive is worth something too!
If it's a Windows 98 computer without an ISA slot, it's not worth quite as much, but people who want to play vintage games want them.
The 2003 era Pentium 4 Windows XP towers are fairly common and not worth much, but prices aren't going down any further. The low point in that market was around 2014 when semi truck loads were sent to recycling. Now it's a little bit hard to find one.
Think twice before pulling some parts out of that old computer that won't be worth much without the computer that goes with them, and dumping the chassis in the trash!