As a general rule, special export taxes or tariffs (as opposed to standard Political Boundary Taxes) should not be levied, although each country has the right to impose whatever level of export taxes or other restrictions it wishes on any of its exports.
Export taxes should ideally be levied on products which the national government may want to reserve for domestic supplies or use for other internal purposes. Petroleum and other mineral reserves are perhaps some of the clearest examples. High-end computing equipment, and other advanced technologies are other examples.
Export taxes should also be levied on products or services (mainly military) which may negatively contribute to political or military stability elsewhere around the world. Such a tax may help slow the speed of military escalations and reduce the intensity of arms races. A ban, rather than export taxes, may be the better option for many categories of military equipment.