Bought a used PC, it fails to power on

Video here: https://streamable.com/n078op

Bought it off facebook marketplace. Tested it extensively before purchasing (3 hours at his house). It seemed fine at the time. No issues during testing.

I got home, plugged it in and noticed it would try to turn on, and then just fail (as seen in the video). Did a clean reinstall of Windows 11. Problem still persists.

Strangely enough, what resolves it is unplugging the PSU power cord, and plugging it back in to the outlet. Then the PC will work just fine.

But if I turn it off for the night, then come back the next morning, I have the same issue with it struggling to turn on, then just failing. I have to do the trick with unplugging of the PSU power cord again.

I can't tell if something is loose OR if it's a PSU issue. I texted the original seller just to see if he was aware of this issue. He hasn't responded so I can't tell if this is bad sign or not. I bought it massively underpriced, so I thought it was too good to be true....and now I may have realized why.

The specs are as follows:

Motherboard: Asus B550M-A Wifi II (comes with wifi antenna)
GPU: Gigabyte RTX 3060 Windforce OC
RAM: Corsair 32GB (2x16GB) @ 3600MHz
CPU: Ryzen 7 5800X3D
PSU: Gigabyte P650G 650W PSU
SSD: Samsung 980 Pro 2TB
NIC: Motherboard - 1Gbps
Add-On NIC: 2.5Gbps