I'm guessing the oiler broke on the original owner an he just kept running it or he was reving it with the brake on either way I'd say that's why I picked it up for 350$
Unfortunately, that wasn't a deal. Fixing it properly is not cheap in parts and expensive in labor. You have to tear the saw completely apart and replace the crankshaft and connecting rod. For a dealer this is not an economically viable repair. THe saw becones a parts donor.
Others can fill in but there was a rash of MS260? MS261s? that had this same issue. Stihl never admitted anything but there was speculation that these crank shafts somehow escaped the heat treating / hardening part of the manufacturing process.
In any case that is ALWAYS an area to check on a used saw to make sure the crankshaft stub is good and not galled/worn. Live and learn.