For the purposes of this post I'll be calling my idea K-marks but its probably a good idea for someone who is either more familiar with Ebberon lore (or just better at making up names) to come up with a better one.
Introduce an npc (probably associated with either the coin lords or house K) who both buys and sells 1 K-mark banknotes for 1 million plat each, these can go into either a collectibles or ingredients bag (does not really matter which imho) and stack to 10,000 outside of bags. This NPC also exchanges different denominations of banknotes for one another, in increments of 1,5,10,50,100,500,etc... all of these notes are completely unbound and can be traded/auctioned freely.
Thus we can store any amount (or at least large amounts) of plat in a very small amount of space. Not only that but we will be able to exchange large amounts of plat with one another, and SSG can add vendors who sell things for very high amounts of plat simply by making the vendor trade K-marks for the items rather than handling plat directly.