A medieval period long-distance trade good needs to be compact, resistant to decay, valuable at destination and refined product. The less a trade good has these four characteristics the less likely it will make a desirable long-distance trade good. In the medieval period, ships or caravans were the only ways to move trade good around. This comparatively small payload capacity to modern cargo ships has severe implications on what can be traded and how far it can go.
If a trader has the choice between two trade goods of equal value per unit, one takes up 0.1m^3 and takes up 1m^3, he will likely choose the former for his journey. He can sell one of the latter items or 10 of the former items. His profit margin will be much higher with the former trade good.
Resistant to Decay
A loaf of bread won't last more than a day before it becomes unfit to sell to someone who isn't starving. Fresh fruit shares the same spoilage attribute. On the other hand, cloth won't rot unless it's allowed to get wet and stay wet. Wood and iron also don't rot/rust if properly protected thus allowing them to be shipped much farther across the world.
Valuable at Destination
This seems obvious but is worth stating anyway. Something that may not be valuable to the exporter must be worth the price to the importer. If not, don't ship it.
Iron Ore vs Iron Bars - Iron ore is bulky and contains ~60% iron per unit of iron ore. While these percentages are high, refined iron ingots are 100% iron so they are 40% more valuable. This case is more a restatement of the compactness point. Aside from the bulkiness, a refined or manufactured product is generally more valuable than the sum of its ingredients.
Logs vs Lumber - A single log is a huge irregular lump of wood that requires special equipment to handle. While some wood importers will want the raw log, many others just want the lumber from the log. Thus, an exporter will be able to charge higher prices and improve shipping efficiency by turning the logs into lumber before shipping them. Also, lumber can be more compactly packaged.