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From manual:

Transactions of products between nations can take place in three different
ways: via the commercial transaction windows for each Commercial
Zone (also known as Land Trade Boxes), via an exchange of resources
through the Diplomatic interface (see Diplomacy), and via the penetration
of the internal market by a third country (see Internal Markets).

Is this really implemented and player can selling his goods directly to Domestic Markets of other countries?

Only half is done currently, i.e the engine calculates if your market is penetrated by others countries automatically, using your commercial techs against their to check that. if penetrated, your internal markets are smaller (because some of the goods are supposed to be sold by others countries) but you can get more from tariffs.

But, what is not done is that a third country don't really sell their goods to the 'penetrated country'... A lack yes, but the game is huge already.

Good to know this mechanic, thanks for answering