I think it depends on where you live. In our area everyone wants egglayers, and most of them have a lot of mutt-ish birds. Many originally get them from the feed stores which use hatcheries. Then the roos in the bunch end up mating with all the different hens producing mutts in the end. So anything "rare" here is anything that is not a mutt. However, people are not really willing to pay much for chicks of any sort. In fact, I wanted some RIR-white this year and even the feed store had NO IDEA what I was talking about, never seen them, never had anyone ask for them...YIKES!
On the other hand, I think if you live in an area where lots of folks have chickens and other livestock, show their animals at the state fair, etc...then I think you have a good chance of selling more distinct or rare breeds.
My suggestion in either case is to raise healthy, clean birds. Know what people want...show, pets, lawn ornamanets, ultimate egglayers...and go with the trend.