c'mon, the tank is clearly not of 'merchantable quality'. that implied warranty goes with everything being sold, even if pre-used unless its being sold on an as is where is basis. I am sure the seller didn't specifically offer our friend with the opportunity of water testing the tank at seller's own place, nor was the tank with water when the buyer inspected. So, the only opportunity that the buyer / poster had to inspect the goods for defects was when he tested the tank in his garage. the defect was clearly patent and led to the tank cracking.
Also, if you see the pics the bottom side is not damaged so I doubt the shimes or uneven pressure had anything to do with the tank cracking. I could be wrong on this latter part though.