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If a unit has a mark for example a possessed squad with mark of tzeentch. Can they purchase the icon of vengeance. It doesnt say marked units cant but im not sure?

Icon of vengeance has nothing that says you can't take it on marked units. Totally fine putting it on whoever you want.

Except Possessed, don't put it on them.

There are some units that don't have it as an option but they are the only ones that can take it.

Yes. All the other icons are mark restricted and reflect so in the very first sentence in their rules. The Icon of Vengeance is purchasable by any unit allowed to do so, under any condition.