Baruca itself doesn't have much customer base, so Ars would need to attract both merchants and customers from elsewhere. If he doesn't collect fees from merchants, it would work nicely for them, assuming sales were good. But if all the customers, in addition to needing to pay for the trip (transportation, food, and bed), needed to pay some entry fees when they return home, it wouldn't be good for them. So, all in all, it would be better for the merchants just to travel to the bigger towns as usual.
Ars would need to found a big industry that also required lots of employees imported from elsewhere. With the growing population, he would naturally also attract merchants. With lower fees than elsewhere, the merchants might actually base their operations in Baruca, attracting even more people to move there.