I think the seller isn't exactly being honest as that appears to be a decent sized crack and even if the boat hasn't been operated in 2 years, I'd have to imagine it's gotten bigger in that time. In I believe it was 1997, Grady White started using a product called Greenwood XL that is supposed to be much more impervious or resistant towards rotting, I'd consider looking for maybe a slightly newer boat or one that doesn't have this as an issue as a new transom would likely cost $7k-$10k+ and take weeks or more to do. I always recommend also having an accredited marine surveyor look over the boat with you and tell you about all of the issues that there are with it. They can use a moisture meter, go over it with a hammer and feel for soft spots, look over and inspect the outboard, etc. If you love the boat and really have to have it, offer considerably less than their asking price as it looks like it could be a serious and costly issue in the near future. If the seller isn't wanting to work with you then I'd run away very quickly.