I know nothing of sheep but since there wasn't a reply yet I thought I would chime in. My answer is about any farm type animal.
I think if the person is buying the animal for breeding, they want a pure bred animal.
If they are buying that animal for eating, I don't think they would mind a mixed breed.
As for pets, most people don't mind a mix breed either.
Mix breeds are generally going to be sold for less money too than a pure bred.
But as far as animals in general, most people want a pure breed.
When you mix different breeds, your not really sure which traits of the parents your going to lose or add.
I don't know about sheep but I do know that different breeds of cattle can have different charachteristics in their meat, as far as flavor & marbling... Maybe look into that? You could ask on www.Backyardherds.com , i think there's a sheep section on there.
You can do a cross with a romanov which will produce a nice meat carcass that matures quickly and also be able to sell lambs for people to raise for meat. This is where you may have to research your area and where demand are. Pretty sure folks on BYH could give you some really good insights on this.
Previous comments are correct that papered sells for more. But there is a catch to this also. Many people would buy a non papered kat because that is the breed they want, it is a little cheaper, and papers don't mean a lot to them. So dependent on where you live non-papered may sell easier and faster.
Well, since I was just shopping for breeding stock this year, I'll give you my opinion. If you want sheep to sell to other people who might be interested in breeding, sell purebred registered stock. I had a terrible time finding registered stock. I would have had to have reserved lambs in the fall in order to be on the list for registered ones. I ended up with some non registered ones (but pure bred). I will be reserving a few registered lambs this year though, for next spring. Many breeders might want to do their own crosses so they will probably be more interested in purchasing pure bred stock to do so.