Do you know that not everyone has access to many different section IDs? You said it yourself, people don't usually bother selling Caduceus, Brave Hammer and King Striker because they figure they are worthless and no one wants them. Even if you pick up every un-ID'd striker you come across, it can easily take many days to find a King Striker. Not everyone knows about The Restless Lion quest either. It can be difficult and time consuming to gather all the pieces and do it yourself, which is why some people are willing to pay for for the convenience to not have to deal with all of the hassle.
And if you know what you are doing, you can make 10 to 20 pds in an hour of farming during HH. Maybe it makes more sense to spend an hour to find a Limiter that's worth 15 pds than it is to spend several hours and headache leveling new characters and gathering a handful of items across different sections IDs.
You know how some players choose to donate money to get DTs to get the items they want, instead of putting in the time and effort themselves? They are paying for convenience. To them it's worth it. That's the demographic that I'm targeting by selling Mercurius Rods.