From the FAQs section:

Q: How can my merchant become a monopolist?

A:

Quote:
Originally Posted by HoreTore
You don't need two merchants, you need two resources of the same type in the same region. Put a merchant on one of them, and he'll have trading monopoly, if there are none on the other resource(excluding your own, of course).

After 5 turns, he'll get the first level of the trait, another 5 turns(10 total), he'll get the second and he'll get the third level ten turns after that(20 total). This doesn't have to be consecutive though, if you have him on the resource for 6 turns, and an enemy merchant sits on the other resource for the next 2 turns, he'll still only need to sit on the resource for 4 more turns to get the second level. Also, the type of resource doesn't matter, ie. he can get 7 turns from Iron, 5 turns from silk and 8 turns on textiles to get the third level, for example. This is why it's a good idea to end their move on a resource he can monopolize when you are moving him towards the resource you want him to trade. For example, if you train a merchant in Rheims and you want him to trade silks in constantinople, you can stop by the iron in metz, the textiles in milan and venice and the iron in zagreb. He'll probably get to constantinople with the first level, which helps a lot in dealing with the merchants hanging around there...

NB:
1. level is capitalist, 2. level is market controller, 3. level is monopolist.