This point has been mentioned, but just to make it clear, you cannot register the seedling you purchase if the grower has kept pieces of it or if they sold/gave pieces of it to other people. So they have to clearly state you are getting all of the seedling to do what you want.
If I saw the phrase "It's slow to increase, so we haven't introduced YET", I'd be skeptical that they haven't already sold or given away pieces of it, or are planning to keep enough to register at some point. Well, if they are planning to introduce it eventually, you aren't going to be able to register the piece they sent to you.
So the description has to be clear that you are bidding on the entire seedling for you to have an expectation that you can register it.
I've bought seedlings from Ginny Pearce (Floragin on LA) several times. She's in a similar zone to me, so there isn't much chance I'll lose the plant over the first winter. And there are a couple in particular that are proving to be great parents.
