A decade or two ago, air travelers were right to worry that terrorists, emboldened after 9/11, might seek to take down airliners with shoulder-launched surface-to-air missiles. That threat has receded — but a new one is emerging from the battlefields of Ukraine, one that’s far more insidious and difficult to tackle. A threat that you can buy on eBay. In the early 2000s, after an al Qaeda-linked group in Mombasa tried to shoot down a passenger plane with an anti-aircraft missile in 2002,