I have a couple thoughts:

On EBay, a seller could sell something, like an amplifier, to someone in another country where they are legal, right? There are a lot of items that have an "illegal to use", which is different from "illegal to possess."

As a forum owner, you would be required to know everything being sale'd on your forum at any point in time and know what laws regulate what items...how would you know that about everything, all the time? For example, a linear being sold on an automotive forum, where there is little knowledge about said linear's legality.

Just other points of view...