If the shop could sell any blueprint it knows, it will probably sell individual ones, probably those found in tech packs, like Hammerhead or Harpoon racks. In other words, if any blueprint the faction knows is fair game, it will probably sell a bunch of junky (and probably known) blueprints.
The OP wants to buy blueprints so he does not need to raid his sponsoring faction for them. For example, he does not want to say, sign up to be Hegemony dog then turn around and raid a Hegemony planet for XIV blueprints because that is "dishonorable".