Incorrect: no substance or structure has ever been stated to be unreplicable.
There have at most been situations where malfunctioning or power-starved replicators have been unable to create certain things; where rules, prohibitions and lock-ups have prevented the replication of things; and where things have been replicated to some degree of imperfection which has had plot consequences.
The closest thing Trek ever comes to declaring something unreplicable is with lifeforms: a type of ribosome was beyond the capabilities of Crusher's replicators in "The Enemy". In contrast, living neural tissue was easily replicated by the EMH in VOY "Emanations", despite the supposed damaged state of the replicator hardware at the time.
We may speculate that GPL is unreplicable or difficult to replicate, perhaps engineered so that slight errors immediately manifest as something you can see, taste or feel (Quark is seen biting his slips at times). But it's not really necessary to go such great lengths. Paper money is easily "replicable" today, yet it is still a practical form of currency - because it's not trivially easy to "replicate", and it can be traced to some degree if absolutely necessary.
Certainly there are easier systems of tracking the flow of money than a physical currency would offer. Many such systems, probably beyond our current comprehension, may be use in the Ferengi Alliance. That still doesn't mean the Alliance couldn't use a cash system of some sort for the everyday bribes - especially since this makes it easy to interact with small-scale foreign operators, often shady ones. And the Ferengi may have a need for that because of the supposed importance of piracy for their economy.
Timo Saloniemi
