There's absolutely nothing wrong with "farm hens." Many of our chickens have been barnyard mixes that a friend caught up from his flock and gave to us. That being said, a bit of research and knowledge will help you out in the long run. Ask if you can see the chickens and check them over before you buy them to see if they look healthy. Get an idea of what some common laying breeds look like, so that you can at least have an idea of what the parent stock of hens may be (if they're crosses). Ask the age of the chickens; some people will try to pawn off older hens that don't actually lay well as young pullets. Know how to distinguish between a hen and a rooster.