Recognize this trading mechanic?

I'm working on a game wherein players may trade commodities with other players and "the bank" -- which represents the rest of society in which the players are embedded. Players produce these commodities on their own using various production facilities that they own, but they may not be able to accomplish everything they want with what they have, forcing them to trade with other players. I've seen plenty of games where players can trade with each other at any ratio they want, but do so with the bank at some arbitrary, very expensive rate. Catan comes to mind as a simple example, but I'm looking for something more sophisticated that does not require calculation.

I've been playtesting this:

Each commodity is represented using a bunch of small tokens that are kept in clear cups. The cups are inscribed around the perimeter with "depth lines" that show how many tokens are in the cup at any given time. At the start of the game the cups are all very full. At any time you can quickly glance at the cup to see at what depth the tokens lie, and these lines have numbers on them that the players will be using when trading.

During the game players use some type of "production mechanic" to pull tokens from the cups in differing quantities. They may focus on being able to pull a lot of tokens from one cup, or tokens equally from all cups, or some combination. When a cup is full, it represents a low supply of that commodity within the community (since no one has any). Conversely, when the cup is near empty, it means there is a lot of supply bumping around, because lots of players have them.

The cups are numbered from high at the bottom, say 7, to low at the top, say 1.
When you trade with "the bank" you use the numbers on the lines as the ratio for the trade. If green tokens are riding at 6 (very low in the cup -- nearly everyone has some) and blue is riding at 2 (very full, almost no-one has them) then you have to give up 6 green tokens to get 2 blue ones (or 3:1). Blue tokens are just more valuable at this time because there is very little supply for your demand. Players are still allowed to trade with each other at any rate they want, but the "bank numbers" are certain to be a guide to the player's choices. If the tokens levels are all kept about equal then trade between token types is about equal, regardless of whether the overall supply is high or low, if the players "produce" tokens equally, or each player specializes in a different type.

But a problem occurs when a single player monopolizes one type of token, say blue, while all the other players and tokens are spread more evenly. The bank registers blue as being in "high supply" so the number will be high, say 6. Whereas the other tokens will be at mid range, say 3, meaning players supposedly get to trade their tokens with blue at around 2:1 -- two blue tokens for one regular token, even though one guy has nearly all the blues and no one else can progress without them. This does not follow reality very well; the value of blue should spike as one player begins to hoard it.

The monopolist can trade at any level he wants. Once he has so much supply that people are forced to get blue tokens only from him, he can ignore the bank's trade level and set his own price. Say, 10:1. (That's fine, there are other mechanics to deal with his monopoly within the game.) The real problem occurs in the run up to the bank running out of blue tokens. The bank actually allows people to trade blue at lower and lower levels right up until it runs out. That's just not right.

So the problem is that I need a good "monopoly trigger" that is quick and easy. Something that quickly recognizes when someone is "hoarding" without any players having to count, divide, etc.

So, any ideas? I have a few, but I don't want to "color" the outcome of other people's genius.
--Patrick