So what if people sell platinum and gold plans for gil?

[cookiecutter]

But player Y may have had other intentions for the gil, like materia. Now that the materia markets is short 5mil, that's unsold materia. Not only that but if player Y wants to get the gil for materia also, player Y will have to gather resources for said gil. That gil has to come from somewhere and it will come from vendors or other sources generating additional income, or selling items to other players, some of which will have bought gil be it for sell runs or expensive crafted items.

There is already people in PF selling the new dyes, the question is, is it cheaper to buy the items directly from the cash shop or is it cheaper to buy the gil and buy these items with gil?

It'll probably be cheaper to buy items directly from the cash shop than to get funded through a gil buyer. I think the current price for one million gil is 5 dollars or so. If people sell one platinum plan for 7.5 to 10 million gil which is pretty reasonable then its much easier to just buy it from the cash shop. Instead of paying around 35 to 50 dollars to get funded for RMT.