Try unchecking 'Show only usable items'. It tries to limit you to things that you can equip, but it gets confused as to whether or not your ship can slot cannons.
That was on of the first things I tried. I've also haven't sold any of the cannons I've put in the exchange. Also had some of my fleet search for the ones I have listed or any. They have had no luck ether.
They are spelt Dual not Duel, so maybe it is a spelling issue. If in doubt drag a common variety of the thing you are looking for into the search box to fill it in for you.
Like I said I sell a dual and dual heavy cannons all the time. I can't even find them if I don't use a search word. You can find cannons but not duals. Sorry for misspelling.
Answers
Still waiting to be able to use forum titles
Not sure WHY but any space station or planet based exchange hasn't these
Use a trade freighter's exchange, the azura, vovwi, and ummm the suliban named thing. Bacon mcbaconface?