Can it a CPU damage the mb?
What do you mean by overheating?
You can check the VRM MOS temps in HWInfo64 - even the most power-crazy boards pushing insane wattage into i9’s don’t get higher than like 60 or maybe 70, so I highly doubt your mobo will get damaged from whatever you’re doing (generally, the auto’s or even maxes in bios are not, generally, capable of damaging your mobo over a short term).
Modern motherboard above a certain quality can put up with a lot of abuse before things go wrong, ignore lemons and situations like that.
I received this i9 from online auction for the cheap price and now thinking about worth it or just sell back.
No it’s just going to throttle
You could run it at 99ºc, 24/7 for 5+ years no issues they don’t care. Like them or not, Intel makes solid products.
I will take a picture bottom surface of i9 12900k soon and upload to it.