Recognize that an issue exists that needs to be thoughtfully addressed. While the seller may be very comfortable with the status of the house component in question, the buyer(s) and their home inspector are not.
Invite the input of an appropriate specialist. It is often best if the buyer and seller can agree on a specialist in the subject of difficulty. The specialist can visit the house to make a diagnosis and also a prescription, if necessary.
Alternatively, either the buyer or seller can choose his own expert. So long as the specialist is credible, their advice is often acceptable to both sides.
Empathize with the other side.
As a buyer, remember that there are strengths and weaknesses in all homes. Understand that “habitability” is sometimes a subjective idea.
A seller must recognize that they may be more tolerant of the weakness in question than other 21st century families. The seller needs to be aware that what used to be a tolerable weakness of their house may have evolved or aged to the stage where action really is appropriate.
In the end, with open minds and constructive coaching from real estate agents and lawyers, the buyer and seller need to agree on a scope of work and/or financial arrangement they both feel is fair.