On the ratling dealer, I agree that there should be a special message if you try to haggle with him. Something like (not wording it the way a ratling would) "I busted my arse getting down here, I'm not giving you a discount!"
On the haggling skill more in general, what if it were usable on any friendly (non-companion) NPC capable of talking? What would it do? 50% chance either way for the NPC to either offer you an item for some price, or look at what you have and offer you some money for something. Would only be possible once per NPC. Higher haggling skills would improve prices in both directions, and would have better chance of more specific information if they're offering you an item (low haggling skill = "Would you like to buy my sword? Just 100 gp."; high haggling skill = "Would you like to buy my mithril long sword of hunting? Just 100 gp.") I'd expect that certain types of NPC would be restricted in terms of what they can offer - for instance, a Child probably wouldn't offer many things, mostly candy, apples, worthless pieces of glass, fluff balls, wooden sticks, and the occasional whistle or flute - but in a rare situation, a child might offer something significant, like an amulet, or a more valuable gem, or a ring.
There could also be a small chance that the NPC will offer a trade of items - "Nice Clean Robes you've got there. Care to trade them for this amulet?"
If the quality of the item being offered were mostly dependent on the experience level of the NPC, it would prevent this from being easily abused, and instead just make haggling possibly a useful skill for PCs seeking to remain lawful (being, in some ways, the opposite of pickpocketing in this regard).
There could even be a few special NPCs that always offer certain things when haggled with. For instance, perhaps haggling with Ruun (when of the right alignment) will give you the chance to get... I don't know, a few holy waters? A ring of luck? Haggling with the mad minstrel could get you an offer of a random instrument. Haggling with Yggaz might result in him offering an item in exchange for a potion of booze, if you have one.