Motherboard-related vibro and bootloop issues

Got a phone from Ebay, from a high-profile seller, listing it as "fully functional", the screen was totaled though. When I received it, I didn't check it's functionality, and got to screen replacement immediately. After I was done, I started it up, and to my dismay vibro didn't work. Insult to an injury - upon plugging in a charger phone goes into a bootloop, and only way to break the loop is unplug the charger. Tried different chassis multiple times, but to no avail, which rules all parts besides from MB. Suspected a software issue, but update didn't help. What could that be, and is there any way to save it?