Those are very common cracks on the corner boxes on the 18s. My 1998 1800 had a similar crack on one side where the box joins the transom when I bought it. I suspect that one is just a crack in the gelcoat and isn't a crack in the fiberglass. If that's the case, its no big deal, just fix it to prevent more water getting in there.
I've ground it down and re-gelocated the crack on my boat twice, and a it just keeps cracking. I have no glass damage and the transom is dry. I've pretty much given up on it and have filled it with 5200 since that has some flex to it. The 5200 has been on there for one season with no additional cracking.
If the crack is in the fiberglass, or if enough water has gotten in there to soak the transom core, then its a problem. I'd bet that isn't the case, but I would thoroughly investigate before buying the boat.
My bigger concern would be why the scuppers are completely underwater in the first photo? Is that boat really transom heavy? The scuppers on my 1800 stay above the waterline even if I'm standing back there.