Trading is an important part of Port Liberty. It is possible to trade in the Wholesale Market of your Town or in Wholesale Markets of the other Towns for arbitrage. An active Royal Sanction does not prevent you from completing Tutorial Mission 5 or buying and selling goods in a foreign town. Try using the Wholesale Market in the partner town. If you still cannot complete Tutorial Mission 5, provide a screenshot showing what happens when you try to use the market so the issue can be investigated.
To trade in other towns you need to travel with your ship which will cost some ship health points due to the long sailing distance.
As a benefit the prices in foreign Wholesale Markets can be better compared to your Home Town.
Each Wholesale Market may buy and sell different goods. Trade over the Wholesale Market is always instant and you can buy as much as you can effort and is in storage.