The exchange (for a resource type) is a track, on which players place little trays (of that player's colour) containing either resource cubes, or coins. The position on the track is the price they are willing to sell those cubes for, or buy them for (and the coins are the payment, as well as giving the count of how many you want)
You can move your trays freely (on your turn; though free for all scramble might be fun to try too); and buy from (your choice of) the cheapest sell bids, and sell to (any of) the highest buy bids.
The bank will always buy a cube for 1, or sell it for 10; except diamonds which it won't sell, and will buy for 5.
And that's the game. Everything else is just a way to get cubes into this system, and create demand so that people engage with it.
Coins are VPs.
Everyone starts with 25 coin three buildings, one in each colour. These produce 0-2 cubes of that colour per turn each. (Random dice roll, at the moment.) You can pay four cubes (of the next colour in sequence) to upgrade a building. Doing so improves the output to 1-3, but also incurs an additional cost of 0-1 of the upgrade cost colour; for a net 0.5 cube/turn improvement. (There's also an option to remove the randomness, sacrificing the income from one building to get average income from the other two.) You also net a prestige diamond. These are worth 20 VPs each, at the end of the game - but are tradable before then.
There is a queue of 5 prestige buildings, from a stock of 10, each costing 6 cubes to build - and rewarding nothing other than 2 diamonds. You can build zero or more of any of these on your turn. On the start players turn, the queue replenishes to 5; or the first one falls off and a new one is added if none were built. If the stock is insufficient, this triggers the last round. Start player rotates backwards.
Finally, you must pay upkeep on held (unsold) cubes - 1 coin per 5 (non-diamond?) cubes; round up.
During your turn; you must do each of these things once, in any order:
Pay upkeep of buildings
Take cubes from production buildings
Upgrade 0-2 production buildings
Buy any, many, or no prestige buildings. (You can buy more than one, but you can't buy one, sell the diamonds, buy more cubes, buy another one etc.)
You can use the buy/sell track as much as you like between before and after them.
And that is, hopefully, it. I think I've got the numbers into an approximation of balance. Expectation is that play is about 6 rounds; and score should be roughly in the range 8-13 diamonds, plus maybe another 1-2 worth in coin; which will usually then be the tie breaker.