I just finished Marauders #20.
In this issue, Marauders try to guess how many knives Storm wears. All participants will owed a "marker" to the winner.
What is a marker in this context ?
Science Fiction & Fantasy Stack Exchange is a question and answer site for science fiction and fantasy enthusiasts. It only takes a minute to sign up.Sign up to join this community
A "marker" is a credit that is owed in the form of an IOU.
(US, slang) Synonym of IOU, an informal record of a debt.
She's basically saying that instead of offering a tangible prize, the winning guesser will get a (small) favour from the others that they can call in at a later date.