How would you say "career fair"? Το παζάρι τις καριέρες/της καριέρας?
This is a very specific term, so it's often kept as is in Greek, too. When translated it becomes "Ημερίδα Καριέρας" or "Διημερίδα Καριέρας". Ημερίδα is an event that lasts one day, διημερίδα is an event that lasts two days.
Just a note on your suggestion: Το παζάρι της καριέρας is grammatically correct, but there is no plural genitive for καριέρα, so we can't say "Το παζάρι των καριέρων" . (and "τις καριέρες" is accusative).