I just went through this 3 weeks ago. I bought 3 blocks (one at a time) for $500 each with a guarantee that each was returnable if not crack free. All three were returned, and I got my money back.
But, I wasted a lot of time and gas money in doing so.
I found so many blocks I couldn't count them all, but all were left out in the weather.
Finally, I got ahold of Vern Tardell and got one from him that had been magged (same price). I then had it magged again, sonic tested and pressure tested.
It was money well spent ($100.00), in my opinion, to be sure of a good block.
Gives me lots of peace of mind.
Good blocks are becoming harder and harder to find.
Prices will be going up