Basically, there are a few kinds of PBN sellers.
1. You ask for samples and they send you ACTUAL domain that has been used for clients. So basically, anyone, absolutely ANYONE can come to forum, ask for a sample and receive a beautiful domain with 50+ niches of previous clients. Amazing.

2. PBN providers who send full reports - This is almost as bad as the first kind of sellers.
3. Ones who send screenshots - As if this was safe or anything. Googling 2 sentences from those screenshots is absolutely no big deal.
4. Sellers who don't send anything - Best option, but far from safe.
You are correct. You can see the backlinks in the GWT, so why not actually let you pick the domains where you want your link to be posted? I think that there is absolutely no harm with that with sellers of type 1 and 2, because ANYONE can see their PBN domains anyway. Regarding sellers number 3 and 4. yeh, this is much better option, but it's far from safe anyway. You either have your own PBN network or you can order from the marketplace. In that case you have to accept a simple fact, that you are already screwed, question is only when.