Could be a lot of things. Could be the FBI runs the site and specifically targeted him. Or they run the site and it has some sort of browser exploit. I'm gues

Could be a lot of things. Could be the FBI runs the site and specifically targeted him. Or they run the site and it has some sort of browser exploit. I'm guessing the first though. That they set up the site specifically to target him. The rest of the investigation involves them running eBay accounts to sell him weapons and such. Seems like the whole investigation was borderline entrapment to be able to arrest him on something.