In the case of temporary, my humble opinion is that you'd be wasting time and money by suing. If there is a valid illness involved, think about how you would like to be treated in that situation.
Is the seller too ill to even respond to the legal claim? Are they even healthy enough to act in their own defense?
Is the seller in a life or death battle? Being so ill you can not even sign your name could mean the seller is in grave condition.
The buyer should consult an attorney to determine their rights should the seller actually not be able to close the deal due to illnes.
This sounds like a situation that may require some compassion vs legal action.
Best of luck to you,
Kawain Payne, REaltor
"If a buyer and seller enter into a valid binding contract and one of the parties dies or becomes incompetent, the contract is VALID if it was signed BEFORE the party dies or became incompetent."--California Real Estate Principles Book, 12th Edt.
Hopefully, it won't need to go to court, but the buyer will be in favor on this one.