This question got me thinking, about the nature of currency and space.
When Qui-Gon tries to buy ship parts from Watto he gets this response.
Qui-Gon Jinn: I have twenty thousand Republic dataries.
Watto: Republic credits? Republic credits are no good out here.
Why are credits no good? Even if they are not a local currency it is likely they could be exchanged. Tatooine is not a lawful planet so even if it was outlawed somebody would buy them at an exorbitant exchange rate.
In the real world, many countries do not have the US dollar as a currency but you can still use it to buy things. Or exchange it for local currency.
The Republic still exists and Republic worlds accept credits, so even outside of the Republic credits should still have value.
It was the largest most stable (up to that point) political entity. So in theory Credits should have more value than the Hutt controlled Wupiupi (I can't imagine the Hutts are too worried about inflation and would mint coins when it suited them)
Yes, space is big, but in the Star Wars universe, it is also easy to traverse, so you can get a bunch of credits and go to a republic planet to use them.
There is, of course, the obvious out of universe explanation that if credits were valid, Qui-Gon Jinn would not have picked up Anakin and the galaxy would be saved.
But in universe is there a reason that credits could not be used on Tatooine?