So I am playing on the FFA server and trying to buy some turrets from the public market at $199... while my private market price is $210. It keeps saying "You purchased 1 Turrets, for $210.
Some of these goods were purchased for a better price on your private market." So I go to the preferences and change it to always purchase from the Public market regardless of cost, and now when I try to buy some turrets from the public market, still listed as $199, "You purchased 1 Turrets, for $210." Sooo.... why is this happening?

Market commission (6%)

$199*1.06 = $210.94, so naturally, you will buy the turret from your private market at $210, which is cheaper.

there is a market commission both from selling and buying units? I always thought it was just for those selling units.

Thanks for the answer.

yeah, it goes both ways