Anyway, you can trade anytime with Duking, after you have visited all three Pokespots. If you haven't played the game yet and are simply planning, it'll be pretty early - leastways after you have beat the first 'boss' (ie administrator - Lovelina) - and rescue Krane. The town you are told to go to next has Duking there, and he'll tell you to go to other places once you find him.