I buy tin in one market town and sell it for 5% more at the market town a week's journey away. Or I load up my ship with tin in one port town and sell it for 2

I buy tin in one market town and sell it for 5% more at the market town a week's journey away. Or I load up my ship with tin in one port town and sell it for 20% more at the port town a month's journey away. That tin can go quite a distance without any one captain having to load up in Penzance and do a long haul all the way to the terminal end of the Mediterranean. But even that would have been possible, if anyone had enough knowledge of enough prices for common trade goods at enough of the ports along the way. One trip would make the owner, captain, and crew rich enough for it to be worth the risk.