Why is my imports/exports always in a minus?

[Koszmar]

Over the course of the game it gets even bigger but always a minus even though I do trade resources.

 

[Noble Zarkon]

You generally have a better economy than the AI so they will be buying things from you which is reflected in the negative number.

 

[elmurcis]

That export/import thing is a bit misleading and has nothing to do with pure resource trade.
It is all your trade routes value (commerce per turn player receive through trade routes) "from" AI(s) minus all trade routes from AI(s) towards you (commerce AI(s) get).

Spoiler :

Limitation for number of trade routes one can get is 1 trade route per AI city (as long as AI is not running Mercantilism = no trade routes at all). (domestic trade routes are unlimited - all cities can have trade route to same city etc.). So that makes late game so negative (many AI's to you but only one you to AI's)
Some things to consider here - if map is "watery" (Archipelago or some other with multiple landmases that might be separated by 1 ocean tile that is possible to cross through own culture), trade routes can be one-sided. In extreme cases I have experienced (free commerce without giving anything to AI - woooohoooo) 150 export (commerce) surplus through that (having "access" to large land with multiple AI's there but none could get towards me (if physically can't get to you, also trade routes can't get. Even if can trade resources as that need only 1 side access to domestic trade network).
Easiest way to detect any issues/changes here is checking F9 export/import value before/after signing Open Borders. And also F4 Info screen with number and value of trade routes to each civilization