They buy duplicates to enhance their collection through trades. Collectors with duplicate keys, trade with other collectors that have duplicates. If a collector sells his duplicates, then he has lost his potential for a trade, not a handicap most key collectors want. Soon you will have a trade inventory of your own. Keys are where you find them.
Some collectors have a special area of interest.
They will trade keys outside that area of interest even though they may not be duplicates. Recently, a veteran collector traded away his only gray base Vibroplex Lightning Bug for another collector's 30's vintage repeating relay. Both collectors were happy. Trading is not restricted to one key for one key. Sometimes there are three way trades involving three collectors.