Sophie, just think please and put a realistic price on your stock. A quick check of the prices for cull ewes in your local mart's website will give you a good idea.
If we as smallholders want to be taken serious by "proper" farmers it doesn't help if we don't know the value of our livestock. You will just be taken advantage of. And buyers should be prepared to pay a fair price for new stock, even if both buyers and sellers only keep sheep as a hobby....
And (in my opinion) it may be fairer to sell a prolapsed ewe as cull, or even fairer eat her yourself, as you cannot make sure that she will not be sold on to an unwitting customer further down the line, perhaps a beginner in sheep keeping, who will then have a hell of a time...