Mike,
The water in the cabin may have gotten in there from several locations or it 
may be that the hatch was left open at some time. The fact that the water 
hasn't drained out is a good indicator that there may not be any 
structural/centerboard trunk damage.
I wouldn't rule this boat out, but the asking price is ridiculous. Obviously 
the owner doesn't know the condition of his boat. A good visual inspection of 
the hull will give you some idea as to the overall shape.
As recommended, check the stringers and everything below the floor in the 
cabin.

Rummy