I train them up in a safe area close to home (one with multiple identical resources) until theyre at the 'monopolist' stage, then move them to somewhere profitable (i.e. usually somewhere far away).

I perform acquisitions if the opportunity presents itself, but don't otherwise go searching for enemy merchants to take over because its just not worth it, either w.r.t. micromanagement or financially. A high level merchant can make 1000 gold per turn, you're never going to make that much performing acquisitions because much of the time there just isn't anyone around to acquire.