postmark vs marker

postmark and marker both are nouns.

Word	Noun	Adjective	Verb	Adverb
postmark	Yes	No	Yes	No
marker	Yes	No	No	No

As nouns, marker is a hypernym of postmark; that is, marker is a word with a broader meaning than postmark:

postmark: a cancellation mark stamped on mail by postal officials; indicates the post office and date of mailing
marker: a distinguishing symbol

Other hypernyms of postmark include mark, marking.

postmark (noun)	marker (noun)
a cancellation mark stamped on mail by postal officials; indicates the post office and date of mailing	some conspicuous object used to distinguish or mark something
	a writing implement for making a mark
	a distinguishing symbol

postmark (verb)	marker (verb)
stamp with a postmark to indicate date and time of mailing