If you profit off of an exchange with someone, it means, by definition, you take more money from the other party than your offer was worth in your own eyes. That is what it means to get gain, scripturally - to profit off of exchanges.
Do you wish for others to take more money from you in exchanges than what they're offering is worth to themselves?
What, then, does the Golden Rule imply about profit?
And for those to whom the Golden Rule remains a mystery, do you love your neighbor as yourself when you see to his needs by exchange, and take more money from him in the exchange than the offer was worth in your eyes? Do you act that way in seeing to your own needs, or do you simply see to your needs freely? How then do you love your neighbor as yourself if you do not do likewise to him?
If you have some other standard of good and evil, what is it that permits you to claim yourself justified before God for making merchandise of your neighbor's needs, and taking more money from him than the value of what you give him?
 Of course not. That's why you shop around to find a lower price, and even then aren't happy with the price you get. And that's also why you try to make a profit in the first place - you don't want to break even, and you don't want to come out behind.
 Profit breaks the Golden Rule.
 2 Nephi 15:20 Wo unto them that call evil good, and good evil, that put darkness for light, and light for darkness, that put bitter for sweet, and sweet for bitter!