I'm sorry, I haven't been clear enough about that feature : you don't get seed when you sell the tree but when you let it alive. It will drop seed quicker than the "grass". When you sell a tree but don't have any seed to plant, don't worry, a seed will appear in the grass if you wait enough, this is a mistakes that will only cost you time. I will add some precisions in the description. I hope you will try again the game.