I spend marks of honor on bonus roll tokens for BfA and Legion content, mostly. I think there’s also recipes by the PvP vendor in Kul Tiras/Zandalar that you can spend marks on. Some potions, other consumables. That’s really it for current stuff.
Otherwise, old tier sets. If you didn’t play Legion, (new) Dalaran has some nice ones for every class…
Donny Kawano backflips into his basement to practice his martial arts. One day… one day, he will be #1. Just you wait!