It's indeed a potential issue. Especially when you want to purchase HQ items but if there is too many NQ items listed, HQ (the most expensive) will be hidden from view.

Technically it could happen even without someone trying to exploit the issue, just with too many NQ sellers.

As for a single individual doing it on purpose, I guess SE could implement a limitation on how many times you can list the same item on MB within a retainer.

And about adding a "next page" button, looking at how slow the MB is refreshing when you try to view different items, I would not be surprised to learn there is a technical limitation on FFXIV architecture which would prevent retreiving too many items listing from server to client.

On a personal practical note, I've never encountered the issue on MB. So I guess k it's not a widespread, recurrent or important issue so far.

Also if someone tries to troll the MB by doing so, that means all his items must be the cheapest, which incendentaly will cause the issue to resolve itself with people buying it quickly. And you can always simply undercut him unless you're trying to sell HQ items (The HQ case being a legitimate reason to be annoyed, should it happen).