If I need 3 of something, I don't buy 50+ on the board. I either gather it myself, or level up whatever job is needed to make it.

The limit of 40 sales at any one time discourages selling small stacks which is why you see so few of them. (I'd rather put up 50 than waste space on 10 stacks of 5.)

Why not let sellers set a partial sales allowed flag on stack sales that lets buyers purchase as many items as they want from it? This should result in faster sales than waiting for some rich player purchasing your 99-stacks.