Before you sign a contract to buy or sell a house

From a post by a Facebook friend:

"I had a call from a friend of a friend. The dear lady is in a contract to sell her house, and needs advice. The buyer has a real estate agent, but the seller doesn't. Unfortunately I cannot legally advise someone once they have entered a contract. So unless you are well versed in real estate transactions, my advice is to hire a real estate professional BEFORE you enter a sales contract."