Yeah, because the traits don't become available for purchase until you're r20, join the guild, and complete the initial quest. Seems to be the case for all DoW/DoM(For sure CON, THM, & ARCH as I've done those)
I have purchased guild mark abilities for both Glad and Thm pre-level 20. So yes, you can purchase certain guild abilities before 20 But...only the ones that are in white(6000 marks each). The ones shaded in yellow for 8000 you have to wait until you are level 20.