Its mostly seen as illegal to use undefined or self issued currency not minted by the government of that nation.   I've heard of a few cases where people just turn out plain metal discs and have gotten into trouble because it was seen as a kind of counterfeiting operation that undermined the tax regime.

If you want to use currency outside of your own countries, its better to take up the established currency of a sovereign nation.    In some cases you can hold a gold or silver coin that is still widely traded and accepted as valid.     I realise some will argue its perfectly fine in theory but in practise it raises eyebrows and some alarm.    Just carrying around large amounts of cash can cause forfeiture for example which is not exactly fair but common as an offfence because of the drugs trade unfortunately