The Antique set is a collection of 5 items mentioned in the quest "Pieces of the Past II". They consist of a Coin, Chalice, Vase, Scroll, and Book.
|Note: The 5 Antique items are not the 5 Artifacts required for the "Explore the ruins" quest. Those Artifacts are inside the Ruins Cave itself.|
All 5 Antique items can be found in the Ruins Area on top of the hill, within the boundaries of the ruined building.
The Antique Scroll is on the ground in front of the bookshelf near the Ferret in the east corner
Other than requiring 1 of each item for the Pieces of the Past collection, the Antique items have no other use.
They respawn a day or two after they are collected, but there is no reason to collect more than one set, because they cannot be sold in the Shop or Discount Bin. Despite them having a sell price, placing them on a shop table will cause customers to bunch up around it, and they will not take anything from the table.