There was another set of sheep horns for sale from AZ about a year ago. They were unplugged and I asked the seller about it.
He stated that since he found them on his property they were his and he could do whatever he wanted with them.
And according to my foggy memory he was right. Now, laws differ by state. In CO, you can pick up sheep horns and keep them - just can't sell them. In MT, you can't even pick them up legally.
In my opinion, you should not be able to sell unplugged sheep horns no matter where you found them.