ihilani - your thoughtful post is appreciated; you bring up good points, to be sure. Remember that this breed was only accepted into the APA in 2011. They are babies, so to speak. For ANY breeders to say "they breed true" is, in my humble opinion, a major overstatement. Again, this is just my opinion. I have been breeding & showing for many years now, and have relationships with folks who are among those who have bred them the longest of anyone in the U.S., and we all are in agreement that it will be many, many years before we really can say that they breed true.
I honestly see almost as much variance in other breeds that were accepted in the 1920s. I don't believe it's wrong to call your flock (and I have not seen them) purebred if you are sure that's what they are. I'm of the camp that if it looks like a Marans, lays like a Marans, BREEDS FAIRLY TRUE, it's a Marans. All breeds are composite breeds; even our beloved Plymouth Rock, which is one of the oldest breeds accepted. To think there won't be variances is just plain silly.
The fact that you are striving for proper type is fantastic - selling hatching eggs, chicks, juveniles, or even adult stock is fine, so long as you are honest. What you state in your post is honest - what more can someone ask? A buyer should do their research and be prepared with questions, and to know what to look for. Many on here (I know Don strongly disagrees with me, and that's fine - everyone is allowed an opinion) think it's purely the SELLER'S responsibility to inform the buyer everything they know about their flocks. My opinion, of course, tell folks where your flock originated, what you are breeding for, what you are working to eliminate, etc. But, the buyer ALSO has a responsibility to do their research and ask appropriate questions. I'll state one more time, this is just my opinion.