Think I figured it out, a bit by random. My stations are entirely self-sufficient, and the foodstuffs was the last part of that. While my managers had plenty of money, it was less than the recommended budget. After all they don't need money when they get the goods for free. But apparently the subordinates still think about money when trading with own faction. I bit the bullet and made sure each manager had more than the operating budget, and now all the subordinates are busy trading left and right.
The weird part is that they had no trouble buying foodstuffs with lower budget from third parties.
So while this makes sense, it would have been nice for the manager to tell me this, so I don't have to stab around in the dark. Also, scaling the budget based on how many are set to "restrict trade" would be nice, there's zero need for a fully player-supplied station to require a 40 mill budget.
So back to topic, has any of this been improved in 3.0?