It is legal to sell a Fairbairn-Sykes in the UK, you have to be a collector or merchant. You can sell a Glock in the UK, you have to be a liscensed merchant, same in any other European country I know. There are rules around those exports, imports and sales to follow. There are rules around the sale of branded parts, e.g. refurbished OEM parts, just follow them. And without details, we don't know why the one offer on eBay was flagged for export. Because domestic US sales are still fine, right?

Again, none of those points have anything to do with ownership of things.

Edit: Also, nobody said anything about the lense sale being illegal, for now we just have eBay blocking international shipping through eBays program, after Nikon US found something with the offervthey didn't like. Just what that was, we don't know.