Many breeders try to sell in pairs to keep their ratio of roos to hens down. My theory about Seramas: they are raised for beauty, not for eggs or meat. And since they are a fairly new breed, type varies a lot. Good pairings will take the best qualities of each to compensate for weaknesses in the other. I would try to pair and sell them as such to make it easier for the buyer to make good breeding decisions.
It's kinda like clothes shopping. Ever bought a skirt and had nothing in the closet to complement it? You end up either never wearing it or pairing it with something that doesn't quite "go". If you bought a coordinating outfit at the store, you'd be set.
My theory is based on an over-the-top belief in the intrinsic goodness, honesty, and generosity of all people, and is generally unsupported by the real world, so feel free to burst my bubble!