"Selling" ID90 travel slots is a sackable offense by any airlines. I know, as I'm ex-airline staff (worked in 2 different ones). Also with the security at airports nowadays with name matching on tickets, it will be impossible for you to travel with a crew ticket (usually a pass or voucher in many airlines).

In the US, this is slightly different as an airline employee (after many years of service) can nominate close relatives or family members/dependents to be eligible for "buddy" pass travel at ID90, domestic and international. They can travel with or without the staff member.