Collectible stamps, or philatelic items, are valued for their rarity, historical significance, condition, and design. They often feature unique or commemorative images and can come from various countries and periods. Factors affecting their value include rarity, condition, age, and historical context, with certain stamps being highly sought after due to limited production or printing errors. Professional grading services assess stamps for factors like centering, gum condition, and color, providing a standardized measure of their quality and authenticity.