I disagree with the comments that there's some kind of conspiracy designed to prevent people from getting the things they need. I have been studying Global Trade for awhile now, and what I've noticed is that there are some items that are always available 24/7, some items that are available some of the time, and then other items that rarely show up.
For example, products that are always on the market are things like sugar and spice, glass, donuts, ice cream sandwiches, iron bars, etc. Items that show up some of the time are stuff like smoothies, grass, animal grain, hats, bricks, etc. Then there are the items that are as rare as unicorns, like watermelon, cream and corn.
If there were a conspiracy, we would see obvious changes in the frequency of these items appearing in Trade. But I myself have never seen that happen. Sugar and spice, glass, watches, etc. always shows up in Trade for me with the same rate of high frequency regardless of whether I need it or not. Same thing with donuts, glass, etc. They always show up for me. Conversely, hard to find items--like watermelon, corn, etc.--are just as hard to find whether I desperately need them or not. I don't see this thing that everyone is talking about where watermelon never shows up when I need it and then all of a sudden, sprout up everywhere in Trade when I don't need it anymore.
So, I dunno. This seems to me to be a classic case of people trying to make meaning out of the coincidences they may have come across. I myself have seen coincidences work both ways. I have had experiences where an item I spent hours crafting because I couldn't find it in Trade show up right after I've finished. But I have also had experiences where I found the exact items I needed when I least expected it. *shrugs*