Unless states pressure eBay in preventing sale of specific items, stuff like this will continue to happen. Also tracking the sale of this stuff is easy considering eBay forces users to use Paypal... easy paper trail for DHS.
In my opinion if you go after stopping sales of one item, would-be-terrorists would go after the next best thing. Spawning a slippery slope of other restrictions won't solve the issue as it'll bring up other crazy ideas for politicians to put into place. While we're at it how about the government requiring citizens to be licensed to own a computer due to botnet cyber-terrorism, how about the government to require keyloggers on all computers in the name of national security, how about all ex-military are considered possible terrorists, how about CCTV in peoples homes in the name of security as terrorists must build bombs in their homes, how about rounding up middle eastern people like the US did to the Japanese during WW2 in fear of terrorists, how about citizens being required to have an ID chip embedded into their skin in the name of security to enter government buildings/travel, how about being stripped naked/cavity searches at the airport and flying naked or Convict Air approved clothing?
Sorry for the rant, once any government gets a power trip they'll go as far as possible until public outcry. Don't be surprised if a government wants to restrict lemons, limes or oranges... acidic fruits can have other uses too 😛