John,
You are wrong on several levels, not talking about papers.
99% (because I don't want to say 100%) of dealers were at the beginning collectors and it is by the way of collecting that they have become dealers, Darcy is not an exception I am sure. Most of this collectors if not all have become dealers to improve their collections before doing it as a living.
A lot of dealers have collections you cannot even imagine. I know a Japanese dealer who has for sale lots of sword Juyo or Tokubetsu Juyo. But if you are introduced to him he will show you for study a part of his collection. Who can say he has been able to study in a row 12 koBizen swords, half of them signed, 12 ichimonji swords, 7 of them signed (and not by the character "ichi"), 12 swords Juyo and higher (ko Hoki, Miike, Awataguchi, Bizen Nagamitsu...).
Concerning origin of blades, I am very cautious as I have seen quite a lot of blades coming from Daimyo collections even with Hon'ami papers having had their attribution overturned by shinsa.
I have been a few times in Japan, visit quite a lot of dealers and attend 3 DTI. I have never seen one of these dealers selling a blade with green paper. The only one doing it (I am aware of) is Tsuruta san but he adds his guarantee of passing Shinsa. I have discussed this matter with people living in Japan and the general consensus was that no blades were "soldable" at the right price in Japan without a valid kanteisho and preferably a NBTHK one.
I won't say I find it strange that a Japanese dealer sells a saijo smith blade without a valid kanteisho (easy for him to get as it is his daily job) but I hope you have a guarantee of shinsa.