Being able to trading various Items sounds like a great idea at first glance, since many have quite full inventorires.
But consider that it opens for various imballance issues.
Examples
Wishing Wells: High ranking/Long term players can help players that they know specifically. Wishing wells can be used to build Diamond mining cities giving a player advantages, since the Diamonds can be used accross the different cities. As such there is nothing wrong with that – but if tradable the players who do not know the high ranking/ Long terms players, will be at dissadavatage having to play to gain/win e.g Wishing Wells. Diamond have a huge effect on your expansion options and space is as most players know, one of the main challenges of the game. So trading Wishing Wells will create imballance.
The option to Trade other type of buildings would also create such imballances. Can you trade to get building from a higher age ?.
Certainly that would create problems for the whole idea of the game progress and tech development. Trading buildings from lower ages would be quite a unnecessary option, evnen a few eras above and any balanced player could turn his or her city into a lower age in no time, due to the lower costs in lower ages.And if one can only trade buildings from the same age, then consider that all these items stored in the many inventories would overflow the market with stuff that essentially nobody of the same Era would really need. Nearly Useless.
For Bost (Potions) it might be slight a different scenario, since they are mostly short term and one time effects. But still, if you are lucky to be in a very high ranking guild, being able to constantly Boost your productions from such trades would give you advantage over players not so fortunate.
As the game is wroking right now
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