Using goldmark isnt illegal, of course.
But having a player in a round that is actively using goldmark to build structures/special units or sabotaging others can really mess with people's experience with the game, there are already many people accusing this game for being pay to win and unfair.
I myself use gold mark now and then, but having someone constantly use it is very frustrating, they don't need any skill or knowledge in the game, they just buy, buy and buy, and their enemies are gone within days.
I think a max. capacity on gold mark spent per round would make the game less pay to win and more fun to play, because even if there is someone who's only thing they can do is spend money, their shenanigans will cease after a while.
Having this feature will make many players happy and feel less saddened that their actions will actually matter, and not nullified within a few hours of gold mark spending from someone else.
Thank you for your time.