When I had Astrophytum in clay it didn't grow and got root mealybugs and I was surprised it survived. The ones I bought here are very young and small and I paid 3 dollars for all of them. I prefer to buy plants that are young and cheap because many succulents dislike being moved around and the bigger and older they are, the less they like it. This is especially true of mesembs.
The person I got these from told me of a cactus she bought for $420 that rotted 7 days after she got it. I would never take such a chance and cannot afford it anyway, I won't spend more than $15 on a plant. They will take some years to get big but they are more likely to survive if they spend those years in one place in your greenhouse.
In general, I have had good experience buying plants through Facebook sellers but I know that there are plenty of scammers so check the reviews very carefully. It is also true that plants grown 350 miles away might be used to very different climatic conditions and dislike the new environment. If you are patient, you will get what you want locally, at the price you want