Because of the nature of this issue and the potential ramifications, you should immediately contact an attorney directly who has experience in real estate litigation issues.
The foregoing is intended to merely provide additional information to the individual posting the question and to anyone reading the question and/or answers provided. There is no specific legal advice provided. Specific advice cannot be given without first forming an attorney/client relationship. This informational answer is not legal advice, and should not be relied on, since it is impossible to appropriately evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue.
You may wish to consider promptly reporting the buyers' claim to the homeowner’s insurer who insured your WI home. Although homeowner insurance policies commonly contain exclusions precluding coverage for mold claims, some mold exclusions contain exceptions and do provide coverage if: (1) the mold was hiding within the walls or ceiling or beneath the floors, or (2) the mold results from a leaky pipe or appliance. It would be wise to read your insurance policy to see what kind of mold exclusion your policy contains; it is usually found in the section entitled Exclusions, under the heading Mold, Fungus or Wet Rot.
If your insurer accepts coverage, it will negotiate a settlement with the buyers and, if necessary, appoint local defense counsel to represent you and your husband if a lawsuit is filed.
If your insurer denies coverage, you may wish to consider hiring an insurance coverage attorney to protest the declination of coverage. Even if your policy contains an absolute mold exclusion (i.e., no exceptions), the attorney can assess whether you can make an end run around the exclusion by arguing that a covered risk was the efficient proximate cause of the damage. In this regard, AVVO's Find A Lawyer page is a good resource.
This response is general legal and business analysis and not legal advice; thus, other attorneys may analyze this issue differently, particularly if there are undisclosed facts. I am licensed to practice in CA, but I am not your attorney and this response does not create an attorney-client relationship between us. Please review Item 9 of avvo.com’s terms and conditions, which is incorporated by reference as it if was reprinted here in full.
I agree with Mr. Navis. It is likely you will not be liable for failing to disclose a problem you had no way to know about. Hence, whether you are liable might depend on the reasonableness of your search before selling.