Possible Clues:

• Might have been licked in the past but won't be licked again - presumably you got that from an old man of letters?
• *Philatelist's find
• Something once sold for pennies that's now worth much more
• Collectible whose price may cause sticker shock?
• British Guiana 1-Cent Magenta, e.g
• Hawaii Missionary, for one, is of interest to philatelists
• The inverted Jenny, notably
• One cannot take it at face value
• Most uncommon map, perhaps of interest to philatelists
• Collector's prize find