Barrel frosting is caused by powder burn. I don't have a problem with barrel frosting if it is even and no major pits in it.

Chambers on the other hand - a big no no IMHO, I would be unlikely to buy a gun with chamber marks in it as if you polish them out they are then out of spec and that can cause other problems.

As to what causes them (chamber marks), case separations, bad cleaning, powder blow back, a number of things.

That's my HO.