Our 1978 (S) 245DL, which we owned from 1992 to 1995, showed evidence of previous rust repairs on purchase and it was again breaking through when we sold it three years on. It was, however, much better than the early 1975 (N) 244DL, which we had for four years from 1984. The tops of the front wings (no liners on the very early cars) corroded right through on that! It might be apocryphal, but I believe Volvo may have used some rather inferior steel in the late '70s.
As 'Burdekin' says, do be careful! Take a magnet on your pre-purchase checks, malbe?!
As to the price: as you say, it does seem steep. I'm no expert, but I would think around half of the advertised price nearer the mark. And why is this chap selling it on after only six months ... ? As always,
caveat emptor.
Regards, John.
Edit: Typed before I read 'Moomoo's excellent advice, above. J.