These secret merchants look like an ol' weapon merchant but they sell around 6 special eidalon items. Each merchant is hidden somewhere on the map so you must explore the map to find these merchants.
For example, If you are lvl 30 and above and know of the GG dungeon, there is a merchant near the gate.
This is the only one i have come across and I recently just found a key fragment for sigran from my Eidalons (the second Eidalon) but its going to be a struggle to get 10 if I or you cant find a merchant that sells the correct Eidalon Pyramidal fragment boxes.
Or, you can just open your map by pressing M, find this secret merchant (they're really called traveling merchants), and autoroute towards them.
They fail at being ninjas :/