Ideally, just like sales listings, buy listings should be fully visible. For example, they can see that MrXtheRetainer is willing to buy 50 widgets for 200 gil each. Next in line is MrsY, willing to buy 100 widgets for 199 gil each. And so on.
But like mentioned... I think this is a good thing. It lets players that don't want to invest time playing the market and tweaking listings get a reasonable value for their goods.
I actually wouldn't mind this, I'd save a lot of time adjusting listings. I'd still like more slots, though...