If the dark line below the cap and above the scupper is a crack, I would be concerned more than if the line is just a stain. There is definitely some decay going on it seems but it is difficult to tell how extensive it is.
If you get to see the boat, check for cracks in that spot as well behind the spot from the bilge as well as looking over the whole transom in and out.
If the problem is extensive, you might be able to tell by lightly tapping with a soft faced small hammer. leather faced is ideal but hard to find. Plastic faced will do. Solid rap sound is good, hollow sounds are not good. If there is extensive damage to the transom, repairs can and will be quite expensive. I would walk away regardless of the asking price if I wanted to spend my season boating and not restoring
Seriously, if you don't think there isn't a major issue, get a marine survey.