Pete,
I’ve never owned a C-50, but I’ve owned 3 different C-30’s. The tube screwed on in the C-30’s.
From the photo of the disassembled parts it appears that the extra chrome ring fits over the tube and the rim of this ring would be tucked under the edge of the gasket to hold the ring and tube in place over the area thats discolored.
I can’t tell for sure whether all or part of the stain is coffee or rust, or a combination of both. At any rate I think that this stain would come off with some gentle scrubbing with an SOS or Brillo pad, and shouldn’t effect the quality of the brewed coffee.
It looks to be in pristine condition. But, and this may be a big but, they didn’t give a good photo if the inside of the lower pot and these can be prone to having the chrome plating wear off, exposing the copper underneath and could effect the taste of any coffee that sat for a long period of time staying warm. I’d ask the seller to send a clear photo of the inside of the lower pot.
Seventy nine dollars is high, but not if the pot really functions properly. The seller should reassemble the tube onto the upper pot before shipping.
Good Luck!
Eddie