IMO, the easiest way to fix this would be to change the URL generated at the top of the table, so that it creates the correct link for people to click on.
Alternatively, you could change the code so that it recognizes the other link and does the right thing.
When you go to a page like http://swrpg.viluppo.net/equipment/weapons/category/1/ and you click on the "Skill" heading at the top of the table, the table should be sorted by skill.
When you do this, the link you are taken to is http://swrpg.viluppo.net/equipment/weapons/category/1/?order_by=skill. Unfortunately, this link doesn't work. If you instead go to the link at http://swrpg.viluppo.net/equipment/weapons/category/1/?order_by=weapon_skill, then that does work as desired.
IMO, the easiest way to fix this would be to change the URL generated at the top of the table, so that it creates the correct link for people to click on.
Alternatively, you could change the code so that it recognizes the other link and does the right thing.