I agree with Michelle. I do not like to sell horses without their papers. I have sold colts on applications, but they always have the option of papers, barring a darn good reason for them to not keep their heritage (dwarf, or serious genetic issues, neither of which I have had yet). I can't do anything about a buyer not actually registering the horse, but I will try my best to give my babies the best chance at good homes. Not that having papers is a guarantee, but it does help. Papers also help with resale value. After all, generally speaking, registered horses do sell for more than unregistered ones.
If I bought an open mare and she was bred, I'd hope the previous owner would help me in registering the foal. It is not my fault the seller wasn't careful, or did not have the mare checked thoroughly (provided this was the case- one of my friends has had several stallions who rather talented jumpers, and she does everything she can to curtail their extra curricular activities). If I sold a mare as open who was bred, I'd give the new owner my apologies, and a congrats on their bonus (as well as the paperwork to register the foal). I think it would be crass and rude of me to demand a stud fee in such a case. However, if it was explicitly stated in the purchase agreement that if the mare came up bred there would be a stud fee due to register the resulting foal, then I would have had forewarning of it (and so would the seller). I have seen such addendums to sale ads on mares exposed, but not guaranteed to be in foal. In that case, you're purchasing a mare you know to be possibly in foal, and you know that if she is, you either pay the stud fee, or have an unregistered baby. Nothing is a surprise.