I have one more argument: I bet that coalition members are less likely to spend gold. Biggest gold spendings happen if a conflict gets to a personal level, in a one-against-one or one-against-many fight, don't they? I can't imagine anyone to spend a lot of gold in a coalition-versus-coalition fight(?)
So since the planned change makes playing in a coalition mandatory, Bytro will earn less.
Anyhow nobody will want to spend gold or even play a game that's dull as a piece of bread.
So if you really insist on removing trade of resources outside the market (which would be a severe loss, as said), please at least also prohibit trade of resources amongst coalition members (between team members on teamplay maps it can be kept up) and remove the 10% tax from the market again.
Then the market would become very vivid and trading would still have some place in the game. And you wouldn't give coalitions this massive additional advantage as the planned change does.
So
* diplomacy would still be alive,
* the game would still be colourful as it would still allow for different ways to play (single, just-share-maps-alliances or coalitions),
* buying gold and thus financing the game would still be attractive.
Now that I came to talk about the 10% tax on the market:
As @nemuritor98 already explained, if you want this to help against players hiding resources if their provinces are being conquered (which I doubt is being done in more than one of a hundred games), you'd have to tax placing offers on the market, not accepting orders. Even if this was corrected: Would 10% keep you from doing that desperate measure?? I figure you only do so if you have no other choice, so you'd do it nevertheless, also if you lose 10% on the way. For a tax to be effective against this extremely seldom edge case exploit, you'd need about 25% tax, which would be almost equivalent to a complete removal of the market.
