From the game files, the trigger for the Monopolist trait line is "TradeMonopoly", but what that is specifically isn't explained. From in-game experience, it only comes up when there are more than one of a specific resource in a given province, and probably only when nobody else has a merchant on one. You don't have to have your own merchants on all of them, just make sure no one else does. It doesn't matter who else is trading silk in Baghdad, just make sure you're the only one on the silk in Constantinople or in Nicaea and it will eventually show up. Sitting a merchant on the wine that's on Rhodes or Corsica, or on the silver in Sardinia will never give that trait, because there is only one instance of each in the province.
As the Byzantines, I used the silk resources near Constantinople to train merchants. As Venice, I'm using the silver and dyes in Vienna and the wool in Naples to train merchants. There are two iron resources in Zagreb and two textiles in Milan, but that area is on a MAJOR road and everyone's merchants run through there, so keeping one surviving long enough to skill up is very difficult. As Spain, I used the gold and ivory in Timbuktu.
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