I would absolutely respond. How do we expect parties to provide us with information if we are not accessible? This is an opportunity to introduce myself and send the engagement letter for expert witness services. If you don't provide that that is fine, you can at least be professional and responsive.
Comparable data, research data, etc are not USPAP classified assignment results. Otherwise, how would appraisers be able to reuse the data.
However; The fact that you did or did not utilize it in a particular project would likely be considered assignment results. The applicability of particular data or non-use could be considered an opinion (remember comp search requests). But, that doesn't mean you can't opine on the (new) attorney's subject transaction in a new valuation or appraisal assignment.
I would respond very carefully. "Yes, like hundreds of other properties, I used the sale of your client's property as a comparable. Beyond that, information regarding appraisals are confidential documents under FIRREA."
I think the more appropriate response would be that "it is property data that was utilized for real estate appraisal projects. The application of a particular transaction to a particular appraisal project includes an opinion of the similarity and that would be considered confidential information by USPAP and Lending regulations."
Then, "I would be happy to engage with you and your client for professional services regarding the subject transaction in relation to whatever legal problem you are trying to solve, lets discuss the issue in general without you providing any attorney-client privileged information and I can let you know what a range of fees would be, what the retainer is, and send you an engagement letter and rate sheet".
or "I do not have any experience in expert testimony or any desire to learn it at this time and therefore, (or I have a conflict of interest, etc); I appreciate you reaching out to me, however, I will not be able to assist you in your litigation related valuation problem"
You can turn that sale analysis into a $5k-10k project if you have the skillset.