Buyers may think they qualify for the mortgage they need to buy your house. They might even have a prequalification or preapproval (here’s the difference).
But if the buyer has been sloppy with managing his credit, or if his income has been erratic, that preapproval might not translate to a loan. A lot can happen between when a buyer starts househunting and when that buyer makes a firm offer.
Secondly, your buyer might have less equity than expected from the sale of her first house. If she bought within the last decade, and used a full-commission agent to sell her house, she might not have any equity at all. If so, she has to rely on savings for the down payment to buy your house. And if she comes up short, the lender will let the deal die.
Don’t waste time with a buyer who isn’t qualified.