When you remove an Armor, Barrel, or Hilt from an item, the Armor, Barrel, or Hilt is Bound to the "slot". So an Offhand hilt is bound to Offhand. You can insert it into a "Mainhand" but then the Mainhand becomes an Offhand.
There is no "vendor" that sells the mainhand. You can find crafters that craft them, buy them from the GTN, or run operations and hope to get the main token drop that you can turn in for the mainhand weapon.