>>149812704
While Doof may have not doing outright evil on the show (as we know about, Hamster and Gretel timeline in the Dan and Swampy Universe is a bit messy) he was selling off inators that he probably made in the past before or after going straight in Phineas and Ferb original series ending. Since we know that not all of Doof's inventions don't last more than just a day.