I'm guessing the OP is trying to purchase through the player's profile. While it will display "purchase," the purchase won't complete for clubs no longer in the Pro Shop.
Clubs are updated all the time. Most of the time, the only differences are in their branding and price and not in the clubs' characteristics. The reason for re-branding is most likely due to licensing agreements or contracts WGT has with the various equipment companies.
The bottom line is that discontinued clubs cannot be purchased. Of course, this being WGT, a while back, that particular backdoor did allow for purchase of discontinued clubs.
This is the way.