Stiltzkin is a moogle who appears at various points of the game beginning in Alexandria. He sells you a collection of items each time you meet with him at a discount price and the items get progressively better as you proceed through the game. Stiltzkin uses the funds to allow him to continue his journey across Gaia, but he will show up at each location regardless of whether you purchase the group of items.
The final purchase that you can make from Stiltzkin will net you a Ribbon but only if you purchase all of the items at each opportunity.
Stiltzkin can be found in each of the following locations listed below. Click on the location to be taken to that section of the strategy guide:
Conde Petie Mountain Pass
The fourth visit with Stiltzkin is along the Conde Petie Mountain Pass and can't be missed.
Cost: 666 gil
Items: Magic Tag, Tent, Ether
The sixth visit with Stiltzkin is in one of the rooms inside Oeilvert part way through the quest.
Cost: 888 gil
Items: Hi-Potion, Emerald, Elixir
The seventh visit with Stiltzkin in the storage room of Bran Bal. You have to enter the room, leave, and then re-enter the room.
Cost: 2,222 gil
Items: Diamond, Ether, Elixir
The final visit with Stiltzkin in back in Alexandria in Disc 4.
Cost: 5,555 gil
Items: Moonstone, Ruby, Elixir
The final reward that you will receive from Stiltzkin (if you purchase all of items that he sells) is a Ribbon. There is no way to go back and revisit some of the encounters with Stiltzkin throughout the game so make sure that you purchase all of the available items from him.