Oh man. You fell trap to the UI monster.
Ok, what you need to do is go to the dropdown menu that filters the categories of items shown in the buy menu, select the 'hidden' category, and rightclick your hidden item back. It will show in the normal list again.
The idea behind this 'rightclick to hide' is to send obsolete weapons and ammo to the UI oubliette so you do not have to scroll many pages to buy what you need.
Also, this preference is saved in the save file itself, so it will persist through everything, except starting a new save.