It is good to see that you are giving the most frustrating part of an excellent game some attention, but I think you are going the wrong direction. It appears that you have tried to slow down the buying. That is increasing my frustration.
Wouldn't a better alternative be to facilitate the seller's ability to know what the buyers want?
I suggest that you add a simple option for the buyer to "offer" a higher price than the HQ or Trade Depot allows. My thinking is that as soon as I would offer a price and quantity for any particular item, the total cost would be taken away from my Simolean total immediately and be lost if I've not made an attractive offer to any buyer.
You would need to either add a new building for sellers to use (perhaps called "Auction House") or add a second room to the Trade Depot.
I believe that adding this functionality would completely solve all the frustrations players feel with the HQ and also totally resolve all imbalance of items in the market. The way it is it is almost impossible for sellers to satisfy the buyers. This functionality would fix that problem too.
Think about it.
Signed: An almost totally pleased Simcity fan.