Some people don't seem to embrace the real problem why people are complaining about this.
I don't mind selling single units of an item, nor do I mind selling stacks of an item. The problem is...I can only sell 20 items per retainer at the same time. And it doesn't matter if it's 20 single units, or 20 stacks...
As they can't increase the amount of items sold at the same time due to limitations, they need to adress this with some sort of option.
Cause if it stays as it is...almost nobody will sell single units of an item...cause it would be a "waste" of market space...but the thing is...a lot of people only need 1 item of something, be it for a quest, or to craft a specific piece of gear or whatever.
Of course overall the system "would" work...but the players won't be using it that way...most will sell stacks...and everybody who is looking for just one piece of whatever will be screwed. This is neither good for the buyers, nor the sellers.
People can say what they want...most people will not be interesten in sellig single units of an item, cause it's tedious to "replace" them every day...when they could just add a stack and whoever needs one of them, buys one of them.
Simple example...to finish the ARM Lvl15 Class Quest you need materials made by a Leatherworker or you buy it from a vendor in Gridania or get the raw material and craft it myself. I didn't want to leave Limsa just to finish the quest and I haven't had said material on me...so I checked the Market Wards...several people were selling what I wanted, but only in stacks of 10-99. I ended up going to Gridania and buy what I needed, but I bought 21 of them. Used the one I needed and put the other 20 up on the Wards as a single unit each. They were bought pretty fast...probably just because of that Class Quest...but I can assure you. Not many people are actually willing to do that, just to "help out newcomers"...