While assuming absolutely zero bad will on your part, I would nevertheless find it fair if you were legally on the hook for whatever happened after the sale, unless you could prove that you provided reasonable means for the users of your extension to perform their due diligence on the new owner of the extension.

This is of course easy to say in hindsight, and is absolutely a requirement that should be enforced by the extension appstore, not by individual contributors such as yourself.

I wouldn't find that fair at all. Bad actors should be legally responsible for their bad action. If I sell you a taxi business, and then all of a sudden you decide to start robbing the customers - it's not my fault is it? And just to be clear, I had no idea if my extension was used for nefarious purposes, but in hindsight it probably was.

How would that even work? What if the seemingly clean buyer sells it to someone else scammy?