I think want they meant is that it's ok to have a character that's a merchant as long as the merchant side actually a tool for personal development and plot. And not NWN monopoly.
So, you plan to destroy a noble house' business, if you want to craft and sell items that pay tribute to a certain god to increase the number of followers, if you want to create some sort of mafia, all good stuff. But if the char's goal is just to become rich by buying cheap and selling it for a higher price, there are better games for that.
And they are not saying its forbidden, they are just saying the char might not be interesting enough to have much attention.
At least that's my take on it.
I think a good ground rule is "would you do this in a Pnp D&D game?" If not, then its probably not good for NWN as well.