From the Wiki :

Merchant Badge can allow Trade Nodes along your Trade Routes provide   vision within a radius, which increases with the level of the Badge. It is also a positive Badge and can improve the Attitude of other Empires.

• Requirement: Earned when setting up a trade route moving through a third-party Empire.
  • Every 5 Resource purchases increase the Badge level by 1.
• Effect: +1 vision on Trade Nodes per level.
  • A Level-3 Merchant Badge can provide   vision within 3-tile radius around a Trade Node.
    
    
    You want the route to pass through a third party empire. Thus the AI with a single territory will easily get it, almost all of its trade routes will pass through different Empires. Being a Merchant means sharing the profits via the "on trade" bonus.

The Wiki is indeed correct on this topic, I have received the merchant badge easily by buying from another empire where the trade routes passe via one or more third-party Empires as well.  

You can simply try to change the mode of transportаtuib for some of the routes. Keep in mind that those badges are temporary and give you only a small advantage, they are nice but not vital to get. :)

I think the requrements for merchant badge was changed after Anne Bonny update. I have started a new game and almost every culture has this badge. In my opinion to get the badge now is enough to have a city and a territory which is connected by a trade route to this city. The trade route without resources is enough.