How far should you go when negotiating the price of something?

I'm an habitual negotiator. I do not think you went too far with your price haggling. You have every right to ask, she has every right to refuse or counter offer. As long as you were nice and polite in your askings, you did nothing wrong. If you did not get the price you wanted, then it's time to look for another dining set. Craigslist has got tons of them, so it should not be difficult.