Even so I did not plan to offer or sell this parts: according to the German gun law are slide, frame and barrel gunparts. All other parts are not covered by this law, like stock screw, magazine sight, springs. And as this parts are no gunparts covered by this law everyone can buy and sell and send this parts using the national postal service like typical products sold from any hardware store.
For us here it seems quite difficult to send gun related stuff to the US because of the US laws. Or to order parts in the US, because of US laws again. Here I need an export permission to export the gun. But that is for handguns easy to obtain and cheap or even free of charge. And importing guns into Germany is easy and free of charge as well. Ok, besides import tax (3-6%) plus sales tax (19%) + costs for the proofhouse. But no expensive import/export-agents are needed.