Mediums of Exchange in Sci Fi

In Isaac Asimov's Foundation, I found the Trader's assertions that they weren't after money interesting. It seems like that IS Hober Mallow's ambition. He ends up wealthy in a swank house with the intention of buying power. His trial made buying power unnecessary but he still ends up a wealthy man. His strategy for peace and dealing with the Seldon Crises was based on simple exchange and trade, but people don't want to trade for the sake of trading. Even traders have an end goal. There is a basic human desire to accumulate wealth as a means to each individual's ends. Mallow takes advantage accordingly. I'm interested in reading the subsequent books to see how civilization evolves past the need for currency.

This also got me thinking about other sci fi series and their mediums of exchange...

Star Wars used Galactic Credits, a fiat currency. From Wookiepedia: "From its inception, the credit was backed by the immense wealth of the planet Muunilinst and the InterGalactic Banking Clan (IGBC). During the Clone Wars, the IGBC backed the currencies of both sides, as it would do again decades later for the New Republic and Imperial Remnant."

The Stargate series centers around the production, use, and trade of naquadah, "a rare, super-dense metal element used by a wide number of different races. Its most basic use is as a weapon: naquadah greatly amplifies energy, making it extremely potent if paired with explosives." I haven't watched enough to know if it was actually used as a currency though.

The twelve colonies in Battlestar Gallactica used cubits even after the apocolypse of their home planet. I found it interesting that they still made bets and would buy things despite the meager circumstances and since it was unlikely the fiat currency of cubits was backed by anything since the colonies were destroyed.

My fave sci fi series Firefly used credits on the core planets and precious metal coinage on the frontier/border planets where Malcolm and his crew spend most of their time stealing and exchanging goods to make their not so honest living.

Star Trek didn't use money. I now want to hunt down the episodes that do deal with trade and the mention of money to ascertain why this is or how they claim to have evolved past the need for currency.