I haven't tried this lately, bu IIRC it works like this: There are two values you need to consider, the amount you want sold, and the threshold. You have to have the "amount + threshold" before any will be sold.
i.e. if you have the "amount" set to 50, and the "threshold" set to 100, then you have to have 150 bushels in the granaries before an offer to sell 50 bushels will be sold. Similarly, if you have the sell amount set to 500 with a threshold of 100, then you would have to have 600 in the granaries before any is sold.