Psu. Gpu. Hdd. The psu is always suspect, you'll honestly not ever know just how hard it was run or for how long. Many series span years, so buying used could see you with an already 5 yr old, hard driven gaming psu that's the same as the new one in the store. Between bitcoin and litecoin mining, heavy gpu gaming usage, idle maintenance etc, the gpu is very suspect, better to buy new. The hdd is cheap enough to not trust to scratched or worn units from 3yr old pc's, better new with warranty.
The cpu is always potluck, but generally they are pretty hard to kill unless you completely hate the pc and drive it at high heat for long periods. OC will reduce a cpus lifetime, but it's not uncommon to see cpus still working after 20 years.
Motherboards are iffy. Relatively new generations should have plenty of life, older gen can have capacitor issues. It'll either work, or it won't.
DDR3 ram is still viable, again, it either works or not.
SSDs are new enough and models get upgraded enough that a used SSD from a newer generation is usually a safe bet.
The case is usually a safe bet used. But much depends on how picky or talented you are. I picked up a fractal design r5 window ($130 value at the time) for $20. It had a small crease in 1 side panel. 5 minutes with a hammer and some 2x4 chunks of wood and no more crease.
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