Apparently the concept of the "galactic credit" predates civilization itself....
What's more mind-boggling is they all take the same form of currency.
Or maybe your credit chip automatically converts acquired currency into some baseline, accounting for exchange rates, etc.?
Of course, we know that in the far future, Republic credits are worthless in the Outer Rim, so why they'd be good in the Unknown Regions/SithEmpire is beyond me.