For cartels? There is a reason that cartels operate in the netherworld of board blocking and double secret trading. Because the stuff they do is shady, undermines seniority, and is generally frowned upon by both the pilots and flight ops management.
Cartel vacation trading really doesn’t fall under that category. I mean, once you get your vacation weeks you can trade them to whomever you choose. I think a lot of guys just have a problem with the way weeks are awarded in the first place, not the trading of said weeks. If someone wants to trade a week with his sim partners cousin, have at it.