It is not a question of haggling, but a matter of attitude. The old man showed his dislike to Rin, and tried to take advantage of her. The price he qouted 300 was for a level 1 potion while she was selling a level 3 potion.
For all that she knows, he could be buying the potion from her for 300 while telling the guild it was bought for 500, and pocketing the change himself.