Thanks for providing the information and on one post.
In my opinion, you, your real estate agent and the Buyer's real estate agent (if there were any) are liable.
Why you may ask? Read on.
In recent years, courts have ruled that a duty of the Seller and the real estate agents to disclose include not only what these parties know, but also should have known. You stated that you never had any water issues in the basement...and knowledge of any water, foundation, grading problems...
While this may be true, this fact alone does not offer you complete protection and immunity.
And you state under Ohio law, you are only required to report on problems you have knowledge of. I do not know Ohio law but disagree with the interpretation by the attorney.
If we look at the conditions that were discovered, it would appear quite obvious that the conditions were not hidden. You as the Seller are considered a layperson. The inspector and the waterproofing contractor are considered experts and professionals. Thus the information provided by the experts would generally hold more weight than the information provided by you. In this case, the inspector and the waterproofer have superior knowledge and experience. The following conditions that you should have known but did not, were not disclosed. These conditons include minor moisture in the basement and improperly graded lot, as discovered and reported by the inspector. And technical problems with the foundation and gutters that were discovered and reported by the waterproofer. You state that these conditions you never would have knowledge of and that would be correct if you did not go to the next step. You would indeed have knowledge of these conditions if you had hired the inspector and the waterproofer yourself and then given their reports to the Buyer as part of your Seller property condition disclosure statement. You did not mitigate your liability because you had the liberty of completing these tasks but did not. Furthermore, you could have been better protected if you sold the property on an "as-is with all defects" basis. In other words, if you had the same information that the Buyer had with repect to those conditions, you provide the reports from the inspector and waterproofer as part of your disclosure. And reference the information in the contract by stated that the Buyer understands that the subject property is being sold on an as-is with all defects basis and the said defects are noted in the inspection and waterproofers reports. The Buyer hereby agrees and accepts those conditions and understands that the Seller will not make any repairs to correct those named conditions.
There is a landmark case out of California titled the Easton v. Strassburger (1984) 152 Cal. App. 3d. 90. Basically the ruling stated that the Seller and the real estate agents must disclose not only conditions that the Seller knows but also conditions the the Seller should have known. You can use those case name search words on any major search engine to get more information.
You indeed are innocent until proven guilty. I do not think that a Buyers attorney woud have a hard time proving a-e. In cases such as these, I always recommend settlement because once litigation starts, the attorneys fees alone would exceed the amount that the Buyer is asking for. Maybe offer to pay a percentage of the total remediation cost with a claim that the Buyer knew of the problems based on the inspection, accepted those problems and bought the property anyway instead of terminating the transaction.