"antiquarian" bookshops implies expensive, bound first editions of serious literature.
those shops are rather called "antyki" in Poland - they sell different old things though, not only books, some pieces of art, some stylish furniture, paintings, different household objects/appliances etc., anything that is old and has also some other values (cultural, artistic, for collectioners)
and a good "antykwariat" should have both 15-20 yrs paperback edition of some Stephen King's bestsellers and some rare editions (in Polish often called "białe kruki" - "white crows") for high prices, just maybe not on the shelves.