The work of Myra Wilkins, an international business historian, is most relevant here. The earliest international business structures were almost entirely made up of extended families and/or members of small ethnic groups. Extended international trade routes in East Asia, for example, were often extended ethnic Chinese families (which explains some.of the large ethnic Chinese populations of many East Asian countries). There were other groups that supported international business around the Mediterranean (Mid-East, North Africa, Europe), often Jews, but sometimes other groups. It was the social control that kept people.in line. In other words, if you cheated the group, you'd lose your entire family, social group and support system.

During this period (pre 1000 CE), they developed a number of mechanisms for safety that allowed expansion of the business networks to mom-family later. For example, Letters of Credit (LoC), which are still in use where electronic transfers aren't feasible, were created in the tenth century. Those reduced the risk of travelling with large sums of currency and had other conveniences. A buyer would give an LoC to the shipper upon delivery that essentially said, "John Doe delivered the goods as promised so when he hands this to you (the seller), pay him what I owe him." It seems odd that the seller sent goods and paid, but this worked because shipments went both ways and sometimes the seller and buyer switched roles.

Once LoCs existed, there was much more control of shippers. It was hard to fence a shipment of spices or fabrics, etc. because only traders had access to good distribution systems. It would have been much easier to steal currency, but shippers didn't get paid with currency. They just had an LoC, which only had value when returned to the right person. So, it was worthwhile to return and get paid. A captain would earn more that way then fencing the goods.

Edit:it's Myra Wilkins, not Wilkens, of Florida International University, but I'll suggest other readings:

An Early Age of Commerce.in Southeast Asia: 900-1300 CE, by Geoff Wade, Journal.of Southeast Asian Studies June 2009 pp 221-265

And there are many options if you look at "Medieval European Trade"

To add to the key point about lines of credit, consider what long-distance trade in the world system of the 14th or 15th Century worked like in the details of exchange. First, for the most part, trade over long distances in valuable luxury goods consisted of a series of hand-offs between a number of middle-distance transportation networks. Say, in the case of the trans-Saharan trade after 600 CE or so, camel caravans from cities like Aoudaghost, Djenne or Kumbi Saleh were sometimes picking up goods that had been carried to those communities from nearby sites of production by a different group of transporters and they would pass goods off in North Africa to transporters who brought those goods either to North African cities or to ships that took them across the Mediterranean or to Egypt and the Levant. At each of those stages of exchange, different kinds of currencies or payments were in play that had little or no value elsewhere--you have to be able to envision an economy that has some degree of global scale but no kind of shared global system of value or economic infrastructure.

Thus, the only point where a group of disgruntled or disaffected transporters might have decided to make off with the goods was at those hand-off points--they didn't have the option to just take the commodities themselves to their final point-of-sale and rake in the money. Tuareg men in a trans-Saharan camel caravan in the 14th Century couldn't steal commodities they picked up in Timbuktu, charter a ship and hop off in Genoa to sell off the proceedings and then retire to a comfortable country home--that caper was impossible in about twenty different ways. In much of the 14th Century global trade, arriving in a destination entrepot as a cultural and linguistic stranger was an invitation to be enslaved or otherwise treated poorly unless you were operating within known and approved trade networks and thus to be treated with hospitality.

And as already noted, those trade networks were often sustained by what Philip Curtin calls "trading diasporas" in his book Cross-Cultural Trade in World History--small communities of mostly-related people who lived as "resident aliens" at the far end of a trade route and handled the commercial affairs of kin who were at the other end of the route. For example, South Asian merchants from Gujarat in Swahili-majority cities in East Africa. If you were a few low-ranking members of an extended kin network and you decided to take all the goods in a particular trading voyage for yourself when you were at the other end of the route, well, it was a really small world you were planning to live in--a couple of neighborhoods of close relatives who had existing relationships with their hosts and who would basically take back what you stole by force and very likely have you killed or exiled. And what you stole generally only had value for exchange anyway, and in ways that were very visible--you really couldn't turn to a back-alley fence who would take it off your hands in return for wealth that was transportable to any number of locations.

The closest thing to what the OP is imagining is really banditry or piracy, which was a serious issue in premodern global trade. Often this was the first thing that an expanding empire or polity tried to deal with, in fact, to provide security and standardization over most or all of the major trade routes through its territory of influence. Many bandits or pirates were disgruntled groups who had been involved in transport but had decided they weren't being paid off well enough and were essentially enforcing their contracts in other ways. But even here, if bandits or pirates did not essentially have their own reliable commercial representatives at one end or another of a trade route (which they sometimes did--at which point the distinction between banditry and legitimate trade gets confusing) what they were typically doing is holding cargo hostage--demanding some kind of locally-meaningful currency or item in exchange as a kind of "tax" on a shipment.

Most of the things that make it potentially profitable to take a few items out of a shipment in relatively modern settings simply don't exist in much of 14th Century global trade. A Tuareg caravan member who quietly lifts some gold from a camel's bag on the way from Taghaza to Tunis and is somehow not noticed in the accounting of goods at the other end had nowhere he could safely sell off or use the gold without immediately raising questions about his possession of it and nowhere to spend it as an isolated individual who was simply trying to live his own life with a few ill-gotten proceeds. By the 14th Century, there's a bit of that starting to happen, but isn't really until we cross into the early modern world economy that this starts to be a more imaginable scenario--and at that point, merchants start to have to think about security, loyalty and law in new ways.

It's not like there was some international police force that would (or even could) track down Venetian criminals all the way to, say, Brussels and arrest them or make them pay up. Right?

follow up question here, were there families rich enough to hire someone to go track down their stolen/missing cargo after piracy/mutiny/robbery/etc. and take revenge for and/or recover the goods?