carpo-, carp- (cerp-) +

(Latin: to pluck, to pick out, to gather, to select)

The change of Latin "a" to "e" is due to the Latin phonetic law according to which in the unaccented and closed radical syllable of the second element of compounds, original "a" becomes "e".

—Dr. Ernest Klein,
A Comprehensive Etymological Dictionary of the English Language.

Don't confuse this carpo- unit with other unrelated "carpo- units":

Greek carpo- meaning "fruit";

Greek carpo- meaning "wrist".

1. Deficient supply, scarcity.
2. Uncommonness, rarity; such as, certain coins, paintings, antiques, autographs, etc.
1. A small and inadequate supply or amount; shortage.
2. Rarity of appearance or occurrence; rarity; infrequency; such as, coins, antiques, books, etc.