Just a block and you don't have the car with it?

If you have the car I'd say fix it, machine it, and build it with the goal to keep it with the car disclosing to any potential buyer that the engine was cracked and professionally repaired. It should not be raced IMO but I've seen some truly stitched-back-together 427 Ford side oiler blocks that had a mile of rod in them run fine in mild street applications (Cobra kit cars). Welding is a valid repair but where there's one crack, there's bound to be more you may not be able to see.

If you don't have the car to go with it you have two choices as I see it:
1. Put money into it to have it welded and then remachine it all, then convince the buyer it's fine.
2. sell it "as is" and don;t waste time or sleep over it.

Personally and as a business - I'd go #2. There are lots of instances where raeally bad blocks were fixed. But the liability belongs to the one that fixes it.