Only the race that owns the building representing the trade good has the right to "license" the use of the trade good to another race. Since you bought the use of it, (and not conquered the planet the building sits on) you don't have the right to license to anyone else, (no piracy allowed)it won't show up on your trade screen. Also, since you and the Dregnin have the use of it, it doesn't show up on their side of the trade screen (only things that one side has that the other doesn't have shows up).
Note: Yes, if you conquer the planet that has the building on it, the ownership rights transfer as well. (which is a difference from GC1, BTW)