It’s a crap shoot, but as long as you can verify the supplier (or at least ensure return/refunds) it’s been okay.
I’ve gotten 2 GPUs and 4 CPUs through eBay, and only one of the GPUs was a scam—still got my money back within the day when it didn’t arrive on schedule (the lister had already deactivated their account)—the RX 6600 is working great so far, and the CPUs have held up in some office workstations and server for a few years now.
I’ve also gotten tons of used ram and used ssds and hdds. I’ve had a few times in my workplace where a few sticks of OEM ram and HDD failed, but haven’t had that issue with any of the used stuff (work or home).
I may be really lucky, or I may be the right amount of cautious (or both), so YMMV—definitely check with others for their experience before you decide to take the risk. Just keep in mind that if you go looking for scams and horror stories, it’s gonna seem like that’s all that happens, and the reverse holds if all you look for is success stories.
I’m not saying I won’t buy high end. Just not new. There are so many used parts in circulation, and some of them in pristine condition. They’re cheaper, work almost as well (if not just as well), and keep e-waste out of landfills/being stripped for precious metals. I get to pat myself on the back and save some money, what could be better?