Say someone need to buy only a stack of 10 of item x. There are two people selling that item. One person is sell 10 of them for 100gil each, 1000gil total. The other is selling 99 of them for a cheaper price, 50 gil each for 4950gil total. Yes the second situation is technically cheaper per item, however for someone that only needs to buy 10 of them would rather just buy the amount they need even at a higher price if it's cheaper overall.

I can't believe people are JUST figuring this out.