Markers (https://docs.oracle.com/cd/E24152_01/Platform.10-1/ATGCommStoreGuide/html/s1401usingorderandcommerceitemmarkers01.html) provide the ability to tag (almost arbitrary) extra data against Orders and Commerce Items.
Note, in all my years working with ATG, I have never come across their use.
Is it possible to use the presence of a marker as a qualifiers for promotions?
It is possible to use them as a qualifier for promotions and I have seen them used on a couple of projects. Note that markers can be set for the following repository items:
So depending on what the rules for your promotion are you could use markers against 1 or more of the above to trigger different types of promotions e.g. Order for order level promotion or CommerceItem for line level promotion.
Adding a marker can trigger a scenario and a scenario can then trigger your promotion.