Max out your zeny i.e., save up to 2,147,483,647 zeny on a particular character and you won't be able to get or add any extra zeny after that. The player in question is simply advertising the Heart Wing at 2.5 Billion zeny. Because they have their zeny capped, you cannot buy said item (the item costs more than 250K, +0 can go from anywhere b/w 500m to 800m depending on the seller) but it also gives them the ability to vend items at a particular price and name the shop as "Add 000 to vend price." This way they say that the item's price is fixed according the vend amount after you add the relevant number of 0's. And no it is not a bannable offense .