Praise be to Allah, The Lord (The One and Only) of the Worlds, and blessings and peace be upon our Master Muhammad, the Truthful and the Faithful.
If the Muslim merchant is honest in his commerce, this honesty is his Da'wah (call) to Allah. If his prices are fair, this will be his Da'wah to Allah. If he is truthful and he does not accept to sell any cancerous materials in his store, this is his Da'wah to Allah.
There is a million ways to reach Allah to such an extent that some scholars said, "The ways to The Creator are as many as the numbers of people". Take for example sodium benzoate (a preservative); the allowed amount is only 3 per thousand, so if an owner of food factory uses 3 per hundred, he will be severely reckoned by Allah, given no one has the ability to know how much he uses, but Allah is The Great.
﴾ He knows the secret and that which is yet more hidden.﴿
Allah knows what you hide, what you reveal and what is hidden from you.