It is an old thread, but for reference purposes: I bought an 2.8C which suffered from a badly affected rear lens. I discovered this much too late, because I had no dark room at the time, nor a scanner, due to relocation. When I found out I could not try to send it back.
In the UK, I was told it was badly scratched and beyond repair. But somehow I did not believe it was, since the glass otherwise seemed immaculate.
So I took a bargain, sent it to John van Stelten (Phocal Point, US) who can polish and recoat it if it concerns shallow marks or fungus.
As it was the rear lens, it did not come cheap at $350, but it has been completely restored. I'll soon have a comparison with my beater 2.8F planar (perfect glass, only the shutter times wheel can be stiff, and deadly sharp).