It's a large merchant that already trained their staff on who he is and what he looks like. You don't grow into a large business that's able to go to the palace by scamming people.
The merchant knows about the stampede, chapter implies the merchant also knows it was a near disaster, and MC implies the merchant has at least heard rumors of the Demon king.
"There is no point to profit if there isn't anywhere to spend it." Sure the merchant could scam the heroic student and doom the country.
Not to mention that until very recently in human history charging more than a good was worth was considered unfathomable. Being an unscrupulous merchant was liable to end in swift death.