Internal space trade
Space is expensive
Moving anything through space requires lots of energy, and energy costs money.
Space currency, like all currency, is something hard to obtain freely, in short supply, controllable, has a trusted value and with greater value as currency than what it is made of. The principles of trade and currency are the same in space as they are on Earth, with one significant difference: mass. Everything has mass and a small amount of mass requires lots of energy to move through the vast distances of space.
Helium-3 is very rare and will be highly valued by fusion technology. Unfortunately, it's a transparent gas, very difficult to differentiate from cheap and abundant Helium-4.
Gold and Platinum are very heavy metals. The expense of transporting them through space will easily exceed their value.
As such, any currency will almost certainly be made of lightweight, long-life plastic.
Trade with an external group, human or alien, will be different. Our money will have no value to them so currency must be something we have in common, something of value to both sides. Rare elements, isotopes or compounds will likely be the basis for external trade.
External space trade
Population I stars are metal poor. Any metal will be highly valued in these star systems.
Population II stars are metal rich. Only rare metals will be of high value.
The "Lanthanides" will likely be the rarest metals in any star system, they include: