My country imports regular clothes, evenv though my own factories don't sell 100%

Bump the tariffs up some and see if the small percentage goes away. It could be the outside sellers have a better price. In the real world that is what would happen, in the constraints of the game's mechanics I don't know. I would assume it is the same.

are you in an soi? what is your prestige rank within that sphere? if you are a sphere leader then i would find this very puzzling... someone else would have to explain i guess.

Just to be explicit: if you are in an soi everybody has to buy from the sphere leader first, i think is how it works. so i could see if you were somewhere down the chain, perhaps you buy a little from a higher prestige producer first before yoruself. i think that makes senese. idk. hope that helped.

Prices you see in trade screen are global. All global production pile up, divided on global demand. After that all factories will sell same % of good world wide.

Buying work differently, you can not manipulate selling, only buying.

Raising tarrifs will makke imported good to expencive for population, reducing population demand on good. It wiill reduce word demand sligtly (unless you playing GB or china, which are a big % of world population)

So, it will have very little effect on how mach goods your factories sell.