Corny rule: under no circumstances expose your intensions ("I really need to sell" or "I really want to buy").
Personally I don't like haggling like on a turkish bazaar. Out of fun maybe, but that always wastes someone's time.
The market will always determine a pareto optimal price, people can live with. IMO sales ethics are important, too. Not only on a professional level, but on a private level as well.
If someone wants to rip you off...he/she gets treated like that, always on a gentlemen's level. That includes speeding up negotioations with a friendly "NO, thank you".
Some people are strange, some want to be smart, some are inexperienced, etc...that is life, nothing personal. Just speed up negotiations.
P.S.: Concerning that young Lady; Making an offer is okay, even on a "not negotinable" invitation to buy. But she hopefully will learn, what opens doors and what closes them.
