Breakin' the Law
Commercial crime
In the countryside, this covered markets and fairs, and typically fell under the baronial or manorial court. In the towns, though, the city itself was the authority, and it typically had one or more merchant courts sitting more or less permanently.
Fraud, failure to pay, even assault between merchants
With the international nature of fairs, the only solution was to have fair courts
Elsewhere, "colonies" were established (kontore). Often demarcated physically, like in Novgorod or the Steelyard in London. Inside that area, that nation's law prevailed. Outside that area, the local law prevailed
Particularly during our period, though, cities were eager to get authority from one prince or another to have their own courts, their own laws. This was a vital tool for gaining independence from the local nobility. This, too, created endless quarrels over jurisdictions, some of which led to outright violence but always led to endless law suits.