Well it says "5% decreased trade penalties for goods/equipments. Also, your profits are marked.". So from what I understand it reduces the penalty you have when selling or buying items (you always buy above the value of an item and sell bellow it). Don't exactly know what the "profits are marked" part means.
Main issue is that you can't skip this perk if you want to be a trading character because how perks work is if you want to pick a lvl 50 perk you have to have the lvl 25 perk also selected, so it makes playing as a merchant character really annoying, since you can't quickly see gear differences in the shop and you have to check them manually one by one.