I dog sat for my dog's breeder a couple of years ago. I thought as a nice treat I would have someone come in and clean their house and found someone through the local paper, they were a crew of 2. After they finished, she wanted more money than we agreed on, and I looked around and realized the value of the 20 dogs and puppies, she now knew where they were, and I paid her more because I didn't want any trouble or retribution.
With my litter, There was only one person I was suspicious of who came over, because of her weird behavior when I found out she told every breeder she wanted their puppy, not anything she said or did to me, and I made sure there were 2 other people there at the time she arrived. On another list I'm on, the breeder said she doesn't let people come over "just to visit puppies" for the future, or to look. I only had one guy do that, he came with his 2 boys, but I thought he was serious, and other than waste my afternoon, I wasn't fearful. It's really a difficult position to be in and a good reason not to have an ad in the paper.