Sounds like a good idea, I would prefer to actually address the matter rather than implement such trade.
I would much rather have CipSoft focus on controlling the entire market, starting by plotting a strategy on how to bring back the value of the existing currencies, by lowering the inflation rate of the internal games economy it should be possible to trade very expensive items with the current in-game implementation.
The issue with that is, unlike many other games, Tibia has no end, and thus the only 'logic' thing to do is to increase the profits of those who are stronger as they'll need more resources. Sadly, this economy model is not sustainable.
I believe I have mentioned in a previous answer. The title of the thesis is 'Macroeconomies of a virtual world', whose author is Stephan Börzsönyi. Which puts in perspective what were the issues at the time of the proposal of the thesis, in 2007. The problem snowballed since, there have been attempts of controlling with different currencies such as tokens and the Tibia Coin, but the main problem is the nature of the game itself, it has no end, making it an unending effort to hold the virtual economy and add value rather than decrease the value of the currency.