The saga I'm involved with is centered in an area where multiple national boundaries intersect and this has gotten me thinking about tariffs/taxes, trade rules, and the natural black market opportunities that exist in such places. In considering these issues I ended up feeling like I didn't really have a solid grasp of what medieval black market goods/commodities might even be. So to that effect, my questions.
What sorts of things did medieval smugglers traffic in?
What sort of taxes or trade restrictions were they attempting to evade?
What level of profit were they looking at, sufficient to motivate the risk taking of smuggling?
What kind of criminal organizations do we have any real knowledge about from the era, and what did they look like? What was their structure and composition?
How were these sorts of activities generally perceived and punished?
Granted, many of these issues are complex and could be very different by region. If it would help refine any information (though general examples are welcome to spur creativity) the Saga in question is located in Hibernia.
Legal restrictions to land trade in 13th century Ireland were generally not enforced. Market rights and staple rights - the typical first kinds of restrictions by medieval central governments and cities - at that time are hard to track down in Ireland and anyway make only sense with established central authority and trade routes.
Sea trade around Ireland is even less regulated on the Irish side. In England and Wales we have chartered markets recorded already in the Domesday book, but they don't control and police sea trade as the Hanse did in at least the 2nd half of the 13th century.
Scandinavian control of sea trade is even less formalized than the English, but may be centralized more directly with the monarch's courts and influenced by the Hanse.
So Irish sea traders have more to fear from pirates than from tax collectors, unless they use the ports of the very big or royal cities abroad.
Irish criminals at sea in the 13th century are more likely pirates than smugglers, and they have to fear retribution rather from the Hanse - if they interfer with them - or some king's warships. 13th century pirates were hard to catch, but if caught should not expect mercy: the sea was silent about their fates.
This relates to part of what spurred my curiosity in that Ireland so fractured that such central authority and enforcement seems highly infeasible.
What are these exactly?
So in essence due to there being no central Irish powers, smuggling is unnecessary and a ship captain is not quite but almost a free agent once they hit the sea?
So these issues would have very different answers, say, in Stonehenge and/or Rhine/Normandy one can presume.
As soon as you have taxes and tariffs that are large people have an economic gain from smuggling those types of gods. As mentioned above markets and towns can have tariffs for merchants to bring gods there to sell.
Another similar situation can occur where roads are limited. An example could be a mountain chain that only has one road through it a river with only one big bridge for a long stretch. Places like that were strategically important and financially important. Because you could tax people if they wanted to pass through that place. So there people would have an incentive to "go around" if possible. Would you risk trying to cross the mountains with what you are carrying instead of paying the toll?
A third situation is if there is a god that is very restricted and limited. Then people would try and smuggle that to the regions where it can't be bought. Think about the prohibition times in the US. In these times I would think that most things that come from the Far East, spice and silk, and those sort of things are strictly controlled by a few people. As the trade is very lucrative. So if you were able to buy the wares before those people did you most likely would need to smuggle them in order for you to be able to sell them. An example would be guilds that monopolize their specific type of trade.
Then there is also god that is stolen. That is also something that one would need to smuggle in order to get it to a place where you could sell it.
Then there are also people who could be smuggled in and out. Depending on where you are there are religious minorities that could get in trouble would be in need of getting away fast, and presumably would be willing to pay for it.
Regarding profits, they will vary greatly. If you are able to get a hold of a highly sought-after spice like pepper that would be more than worth its weight in gold to some.
A concert example of tax or tariffs would be during the Industrial Age. It would then cost more to ship cotton on the canals from Liverpool to Manchester than it did to ship it from America to Liverpool. This was due to so many people owning stretches of canal and each one of them having their own tariffs you needed to pay in order to use that part. It's not age-appropriate, but it was the best example I had.
Sorry if I am not able to help with more specific examples. But I hope you can find something to help you a little at least.
The strongest example of trade regulation I am aware of in medieval Europe is regarding the Livonian Crusade, when the Pope decreed that the port of Semgallia should be avoided and Riga used instead. According to the chronicles of Henry of Livonia merchants gleefully enforced this rule, and yet the port of Semgallia remained open and profitable.
Those with lesser authority than a pope would have had even less success.