I feel when you have an as is where is sale, you should do whatever in your power to inspect and verify what you have prior to completing the transaction. The seller should have said, "I was told" it had such and such, then he wold be totally off the hook. However, I do feel like he owes you one Detroit locker, because he states that in the ad and it is not readily available information from a visual inspection.
I understand you could have opened the difs. But carry it a step further, if he had said flat top pistons, would a buyer tear a motor down before purchase, not practical. So, in my opinion, he covered that up, honest mistake or not.