Don't waste valuable marketing time on a buyer who can't get a loan. Pre -qualifying interested buyer's is essential and it is something your banker or mortgage specialist should be able to help you with.
For example each time I have sold a business, I've always asked for a letter from the potential buyers banker, before releasing commercially sensitive information I would always ask for an assurance that the buyer has access to sufficient funds to purchase the business. I would always say to the buyer, "I want proof that you have the ability to buy my business."
Selling a property is somewhat similar. It is imperative that you pre - qualify an interested buyer. It would be stupid to let someone who is not able to get a loan, tie up your house sale up with a contract that is never going to be finalized. That would be a waste of everyone's time and could potentially lose you a genuine sale by a genuine buyer.
A mortgage specialist could give you guidance as to whether a buyer can afford to buy your house. They will be able to tell you if the buyer would qualify for a mortgage and if it is worth proceeding further with the transaction.