When buying used, pack up a suite of benchmarks and tests and learn how to use them. They will tell you if a part is actually good. If tests say yes, then you most certainly will be fine with the part.

By the way, I'm typing this with a used graphics card. It was probably decommissioned off some mining rig, IDK. RX580. The sweetest GPU I've ever had. Love this thing to pieces.

There's one important exception to it though:

STORAGE.

Disks DO degrade over time, both HDDs and SSDs. SSDs degrade as they are being written, HDDs degrade because HDD gods say it's time for it to go, who the fuck knows, they are electro-mechanical devices, they got a lot of moving parts.

Buy used storage only when in a pinch. Because you don't want to know what happens when a disk you store all your life's work on gets degraded. Buy new storage if you can - preferably double, because you have to keep backups.