I never had too much trouble with this because money has high overhead. If you only have a small number of items that are scarce enough to be rival goods (plane

I never had too much trouble with this because money has high overhead. If you only have a small number of items that are scarce enough to be rival goods (planets, starships, artisinal labor, concert tickets, whatever) then you will probably barter for them, often in favors and debt. Ian M. Banks does a good bit on this in one of the Culture books (Look to Windward, I think) where the Culture "reinvents money" in the form of IOUs over access to a particularly popular live event. I did have a problem with the retconning where the Federation was never even partially capitalist at all, because The Original Series is full of allusions to the fact that it is: An immortal being (Flynt) who buys his planet outright, miners who make more profits once they start working with the Horta, etc., etc.