You need to: buy and equip dickwear, buttwear and mouth gag, then go to Merchant again, check wares again and then the scene will play out. Do not skip it because there may be skippable checks in it.
Also game might just be bugged for "Prefect toy" and "Locked up" achievements. In my case, I could not get "Locked up" achievement while trying several times, even picking Paladin class, getting requirements at the town and talking to Merchant right after. Maybe autoequip messes with the scripts like it does with codpiece quest.
