I whanted report a bug on currency for actual ECO version 0.7.3.1 beta.
It's when the leader of the world set negative taxe that cause bug.
When someone buy in your store with a currency who havn't Tresory, the negative taxe agree him to pay less money.
So when no Tresory in the currency , if people buy for more that they have money, everything become free for them.
Ex: Player A have the store. And player B have 50 $. A glass is selling 1 $ in the shop.
If player B bought to player A for 60 glass (so for 60$, more money than player B have) so player B take the 60 glass free and give no money to player A.
So negative taxe in a currency who havn't tresory make that bug when player try to buy more than they have money.