(edited by KlNKl.8094)
Cannot salvage Invaders weapon?
The only way you can get the sigil is to transmute the invader weapon.
In the last patch they made it so purchased weapons cannot be salvaged.
https://forum-en.gw2archive.eu/forum/info/news/Game-Update-Notes-March-26-2013/first
Anything I post is just the opinion of a very vocal minority of 1.