Can the printers be regulated to refuse to print weapons, or could new designs simply circumvent existing prohibited products?

CNN: 3-D printed guns are a boon for criminals

So in 1989 the international sale of new ivory products was prohibited.

FORBES: When You Ban The Sale Of Ivory, You Ban Elephants

For years, agreements that retailers have signed with third-party payment processors prohibited merchants from selling products at different prices depending on payment method.

FORBES: Credit Card Users May Pay an Additional Fee