First off, I think you have to want and like chickens yourself, as you are never going to make a bunch of money selling chicks or chickens. The best you can hope for unless you have show birds, is to break even. You my make some in the spring/summer, but you won't sell many in the fall/winter and the chickens you have need to eat year round, so they will eat up anything you make in the good months.
That being said, P.O.L sex link pullets are good, they start to lay earlier, and lay lots of very large eggs for a couple of years, after which time your buyers will need new pullets and you can take back their spent hens for the soup pot. I also think that for most backyard flocks peope like the brown egg layers, with maybe some blue/green egg layers just for fun.
I for myself like a flock of several different breeds with lots of pretty colorful birds. I still don't have all the breeds or color variations of breeds that I would like, but maybe some day.