Because it's possible to buy and sell member tickets on ge with ironman accounts as well as to trade member tickets to ironman accounts then it's possible to buy a member ticket with a non-ironman account, trade it to the ironman for free, sell it for 1 gp less than the lowest price was on ge and buy it with the non-ironman account on grand exchange (and repeat only losing 1 gp overall, because the main would only buy it for 1 gp higher price the very first time). Which will mean that one can transfer coins to their ironman.
That may sound confusing. What I mean is that let's say that the lowest price for member ticket is 10m. One can buy a member ticket for 10m with their main account, trade it to ironman, put it on ge for 9,999,999 coins on the ironman. Buy a member ticket for 9,999,999 coins with their main (or let anyone else buy it) and the ironman gets 9,999,999 coins from nothing.
People could easily transfer billions to their ironman with this, which would kind of ruin the idea of ironman accounts.

I would propose that ironman accounts shouldn't be able to sell member tickets at all. Not sure if that can be done without removing the ability for ironman accounts to buy member tickets on ge but I don't think that it should remain how it is now.