Can someone back out of a sale if a deposit has been made?

They are not supposed to. you didn't say what it was but let's assume it was a home sale-you may be able to sue for breach of contract but no one can actually force a seller to sell that has changed his mind and although it doesn't happen often, people become attached to their homes and sometimes get remorse so stop a sale. it is actually easier for a buyer to change their mind-they have many more outs.