Put a tax on sale prices (merchant-side) and based off the guilds present in that town, Example: Weavers guild in Uldah results into no o low tax for weavers merchants in the ward, while an armorer wich has the guild in Limsa will see it's sales tax present or raised. this way you get an even distribution of people in every town and incentivates people to move to other towns to buy/sell stuff, while at the same time offest a little with the undercutting due to people potentially losing gil if they lower the prices too much. Tax rates can also vary based on the items availability putting some sort of "control" over undercutting prices.