I had always assumed that "no money" did not preclude having (electronic or virtual) currency. Within the Federation, currency would not be a commodity in itself, but that individual would still be credited for their labors and contributions. Within larger governments (first, United Earth, second, UFP), currency could exist as a measure of exchanges on an abstract level, as along as all the members agree to standards for earnings and exchanges. It would only be a problem when UFP citizens made contracts with those from other governments. The only time money was used was on DS9, and even then it seemed that Starfleet personel were not eager to conduct business in "latinum," but preferred to exchange materials.