Hi Murfasaurus
You seem to have a power, loose MB connection, or a hardware problem. You did find RAM that needed to be reseated. Other required power supply connections could be loose. Check the AC power cord connection to the power supply. Connect the PC to a different AC outlet.
Check or reseat the 24 pin and 8 pin power connections for the MB and CPU.
This product is slightly over a year old. So it is still new in my mind.
I would take it back to the seller. Let them fix it or get a refund if this is possible. Did the seller give you a warranty?
Or you could go through the time consuming task of troubleshooting to find the problem.
The MB has onboard graphics. Pull the discrete graphics card. Connect the monitor to the onboard graphics.
See if you can get the PC to POST.
No mouse and keyboard LED activity is not a good sign.
Any life on the tower (HDD LED, power LED, HDD spinning, CD/DVD drive initialization)? Any CPU or system fan activity?
You would have to test all components installed in the chassis to find the problem.
Grzy