Passengers need to be able to get to the market within their journey time tolerance. It is not clear from your description whether the issue is that there are no goods for the market to sell (in which case, staff will be laid off until goods become available) or that there are no staff (in which case, visiting passengers will not be able to visit the market). You might find assistance in the passenger destinations display in the minimap - look for the colours of passengers who try and fail to reach the market from nearby towns: dark red means that there is something wrong with the market itself (e.g., no goods, or, so far as visiting passengers are concerned, not enough staff); pink means that the journey time to the market is too high and orange means that there is no route.