I'm not saying items are being hid. I don't think they are. I was addressing the notion that an algorithm that selectively hides items from a particular city based on the city's current demand would be complex or slow. It wouldn't be.
Your subjective experience tells you that items are not being hid, and their availability or unavailability is random. Others claim to have observed certain time-intensive items disappear from the market as soon as there is demand for them, only to reappear as soon as demand drops. Each is accusing the other of imagining things.
I go back and forth. Sometimes it seems like the selection of items on the market is not random, because I see a good mix of items instead of wall-to-wall donuts and Vu stuff. So rather than hiding items, perhaps there is logic that improves the mix of items. But that can also be explained by people changing their selling patterns.
I don't know about time-intensive items, but I am sure that expansion items are not being hidden or suppressed for higher-level cities. However, it does get harder to buy expansion items on higher levels. I have two cities, one at level 26 and one at 14. I achieved full land expansion in both, but was able to do so much faster in the level 14 city. While expanding, I was intensively shopping for dozer parts in both cities, and was generally able to make a purchase 5-6 times more often in the lower level city. But not because items were being hid in the level 26 city -- on the contrary, it was because the level 26 city had many more items unhidden, crowding out the dozer parts. Basically the market for specialty items gets diluted the more items you unlock.
Running two cities in parallel at different levels allowed me to observe some cool things about the market. I would put something up for sale in one city, and immediately refresh the market in the other, and almost always saw my seller city in the first few slots. If I hesitated a second before refreshing, my seller city would be much further down, sometimes not there at all -- that's how dynamic the market is. There are at least 10-20 new items advertised every second, sometime much more than that.
And the number of fingers pecking at dozer parts is huge. There were many times when I would tap on a dozer part, travel to a neighboring city, and see several other expansion items for sale. Ok, time to start pecking. I learned that it pays to go for one of the unadvertised parts first, because usually people make a grab for the item they came for. So I'd tap an item and get it, tap a second and get it, tap a third -- here come the llamas. I had a very real sensation of fighting over a handful of items with 20-30 other furiously tapping mayors.
I think the market has improved in the last couple months. Not sure what they did, but it's working for me.