Marks it "Collectors' items" (6)

I believe the answer is:

stamps

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But I can't explain it at all! Could you help me understand?

(Other definitions for stamps that I've seen before include "Philatelists collect them" , "Imprints" , "Brings one's foot down heavily on the envelope" , "Brings foot heavily to the ground, as in anger" , "Philatelists study them" .)

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