Reply #1: We already have a universal currency

[Xipe Totec]

In fact there are three freely convertible currencies in the Galaxy, but none of them count. The Altarian Dollar has recently collapsed, the Flainian Pobble Bead is only exchangeable for other Flainian Pobble Beads, and the Triganic Pu has its own very special problems. It exchange rate of eight Ningis to one Pu is simple enough, but since Ningi is a triangular rubber coin six thousand eight hundred miles along each side, no one has ever collected enough to own one Pu. Nigis are not negotiable currency, because Galactibanks refuse to deal in fiddling small change. From this basic premise it is very simple to prove that the Galactibanks are also the product of a deranged imagination.

- Some Information to Help You on Your Way,
from the Hitchhiker's guide to the galaxy.

http://www.acc.umu.se/~ola/hitchhik.htm