You say a trading tax will increase the price of trades, well don't you think that a troop tax will increase the cost of adventures and therefore the cost of loot spots?
More importantly a tax of any kind will actually make many leave - my suggestion re traders was purely to speculate that I think its the hard core trading community that accumulate the coins and impact the market. There's many things BB could do and everyone of them will impact the game in some direction.
I would suggest its more about the age of the servers, the longer they exist the more coins and wealth they generate leaving those that start much later disadvantaged unless they find themselves a niche, such as hard core trading, selling of Loot spots from spewing out dozens of Roaring Bull or FairyTale Adventures every week.
Give it another 12 months, and if the game is still going, granite will start coming down in price as many more hard core players have all their island at L5.
And whilst on the subject of server age, I think back to the beginning, people thought a fair price for gems was 3 coins, that made it a lot easier for people to buy gem giftable items, now its 20 coins or more - I often see Nobles being sold for 2k+ coins these days. So whilst you may think coins have become devalued, others will say its for gem giftable items the its harder to buy good stuff with coins for the newer players. So how does taxing anything, troops or trades help that situation as some people will say that gem items should increase even further making the divide between new and old players even larger.
So in summary, I am not in favour of any sort of tax. Offer players the ability to buy non tradeable expensive items for vanity and status, that will take gold out of the game quicker and have negligible impact on adventures or trades. I can imagine a situation where I would be happy to throw 50k coins away to have a statue that doesn't impact my building spaces but says hey look at me I can afford this. Then I may want one at 100k, then 250k, then 1M. Yep, whilst the game isn't player against player there is a status thing that people will always throw money at........