Since v1.3 and the implementation of the progress bars for combat, trading, and exploration rank, I've noticed that I seem to get a bigger increase to my trading rank from bulletin board missions than selling the equivalent profit worth of goods through the commodities market. Please note that I said it seems to me to be bigger increase but it might be the same. But I can confirm that I have seen my trading rank increase through the completion of bulletin board missions, especially those that give you the goods to take to another location, including the smuggling of illegal goods to a different station/outpost. I can confidently state that bulletin board missions can increase trading rank because for a long period I was doing missions without carrying extra cargo to sell in the commodities market and my trading rank progress continued to increase every couple of missions.