Those icons in the market are for selling 100 of selected resource, and buying x desired resource.
The resource traders win is selected from the coins icon, either in the market or the upper panel.
Default is 15% W, 15% F, 35% S, 35% M, or something like that. You change that as desired. That is why you see the trader earning x resource on a trip, x other resource in another, according to percentages and distance.