Hey Brian, to set the scene, I have an archipelago if islands that are the centre of the world's commerce.
I use coins to some extent, in terms of paying cabbies, fares on passenger vessels and rooms at an inn and so on. However, I have a central exchange in the capital that publishes weekly values for common commodities, values that are based on demand. Realistically this merely provides a basis for haggling in distant places (where the information would be weeks old). For larger purchases, the Exchange also issues paper bonds to the value of X (which, in the future, would no doubt lead to paper money). These bonds are infused with signature 'magic' which ensures authenticity, so they cannot be forged (this does not, however, mean they cannot be stolen).
If people are trading gems, jewellery and so on, they would need to obtain a writ from a registered appraiser and then take that writ (along with the jewellery, of course), to the Exchange, where they would give the bearer 90% of the value appraised, with 10% going to the appraiser. Though this sometimes means that appraisers might up the amount to get a greater cut, the Exchange has its own appraisers to validate the writ. If an appraiser is pulling a fast one, they'll get black balled, stricken off the register and therefore unable to work their trade at all. This is the same for ship builders, and so on.
It is possible for someone, if they were to have a large bond, to live off it for years. When they need money they can lodge the bond with wherever is local and receive coin to the next lowest denomination of bond, thereby being issued the coin and a new bond. Throughout, the Exchange will take its cut, however.
Phew. Having said all that, I tend to shy away from details like that unless it matters (i.e. they're starving and have no money). Though I delight in Kvothe's penny pinching antics, for me it's a can of worms I'd rather remained shut.