They get Monopolist for sitting on a resource when no hostile merchant is in the same region on a resource only.

Merchants can get following good skills:
NaturalMerchantSkill (oncreation)
GoodMerchant (takeovers)
SecureMerchant (stopped takeover)
WorldlyMerchant (expend movement)
Monopolist (alone in region)
MerchantsGuildTrained (from merchant guild in town)
MerchantsGuildMember (from master or grandmaster guild in faction)