I think that what we are seeing is that the game is showing us primarily items that are being sold by other players who are at our same level. I just went up a level and found that my Global HQ was full of the new items, which it hadn't been before I went up that level.
I've had the best luck by clicking on items similar to what I want. Often, people post an item and advertise the sale, then start adding similar items for sale that they don't advertise because they are waiting the five minutes. I keep refreshing the trade depot to see if the item I want will come up. This method works more than repeated trips to the Global HQ!
