An idea I've echoed in another thread that might offer better viability to open market trade would be expanding the total values of the markets, and tying that total value expansion to vaster ranges of deficits and surpluses as the game progress. Or potentially tying deficits an surplus amounts to Cargo Hold availability.
Deficits and surpluses in the markets, in my opinion, are too small as the game progresses. Buying 200 units of surplus early is fine, but as you progress from early-mid game unit numbers like that become woefully lacking in any real meaningful dedication to earn profits.
As a early to mid-level player the deficit and surplus mechanic became incredibly unattractive and the farther I progressed the less that mechanic seemed to be relevant.
As I became a major distributor though founded colonies late game, checking on the markets revealed that it appeared as if my exports were flooding the market with surpluses, and almost no deficits to be found anymore. And the surplus numbers were, again, quantities ranging from 200 - 500 units: irrelevant quantities for a fleet carrying in the thousands.
That really doesn't make any sense. One way or the other, if I was exporting enough goods to flood the market the unit range of surpluses should become higher, but also I shouldn't be exporting enough goods to do that as all of my colonies grew to become large population centers, which made them massive consumers.
So I definitely feel trade could be improved in concerns to open market deficit and surpluses. And probably would be best done so by expanding the total value of markets, possibly as the game wears on or as additional colonies are founded, thus increasing the range at which deficits and surpluses happen.
