It will always be possible to cause harm with legal objects, unless they ban fists and teeth.
While I think linking terrorism is a stretch, there are benefits to regulating the sale of things designed specifically to cause death, automatic firearms, high capacity magazines, warheads...
Which goes back to my original point. Linking terrorism to OpenBazaar is a far stretch and is rooted in fear, not data.
[1] https://en.wikipedia.org/wiki/Boston_Marathon_bombings