I think diplomacy would be a good time to introduce a little bit of actual trading.

Something like a team market so you can sell and buy resources directly from your teammates.

The Price should be about 50-75% of the black market price with 10% that "vanish" in the progress as "tax".

You could do it with a cap like thing "all above x go into the market" so someone can not buy you dry.

Or like x% of my stock (in cr, crystal or metal) are free for trading with my teammates.

I think the resources should always stay at your disposal not like the direct selling now that those resources are separated.