Each route must include at least two markets and must specify transporters that allow travel from each listed market to the next and back from the last to the first. At each stop, you specify what commodities should be carried to the next. That implies that a merchant who reaches that stop will wait to sell carried items that are not required to be carried on to the next stop and will buy enough to reach the specified number of items to take away.
When you click the "transporter" icon, you see a list of other transporters that could be used at that stop, each with their capacity noted, and the hint text gives you more details. A game option lets you see transporters that are not currently available to proceed from that stop but would be if there were some improvement such as a depot.