Or maybe, humans simply do not use currency with one another. You can get a beer (ok, earth ale) free, but the replicator stinks or you can get an imported andorian ale, but you need to pay for it. The betterment of themselves stops at the first bar that serves non-human drinks.
One may wonder if the Chateau Picard is made primarily for the export market.
Nonetheless. even in the contemporary world, where almost all currency has a symbolic value, transactions between individuals requires that their respective governments have adequate financial relationships: balance of payments, currency reserves, etc. In the ST world, larger business dealings may not be so problematic, as the Federation could easily keep a reserve of gold pressed latinum to satisfy the Ferengi government. A small businessman, like Quark, operating an auction between peoples of numerous cultures would likely be less willing to rely on symbolic values and prefers to be paid with something that has an intrinsic worth.