And seldseen costly stones of so great price— Marlowe, The Jew of Malta
... and awakening, saw a serpent like a dragon, a seldseen sight,—The Arabian Nights
Etymology
From Middle Englishseldsene(“rarely encountered, seldom seen; rare; as adv. seldom, rarely”) also seld-sēn, seldscene, seltsene, Old Englishseldsīne(“rare, extraordinary, unfamiliar, uncommon”) also seldsiene.