Maybe I just don't get the whole "garage sale" aspect of CL, but to me, when you agree on a price via email or telephone beforehand, it's really tasteless to offer less at the time of the transaction UNLESS the item is seriously misrepresented.
A couple years ago, I was selling a Maclaren stroller. It was in mint condition- absolutely perfect. My asking price was 50% of what they were going for on ebay. My CL ad said that my price was firm- no offers. 8 people emailed about it- I went down the list and the first person agreed to my price, we set up a time, etc. She stood on my front porch and hemmed and hawed for 30 minutes- then offered me $20 less than what we had agreed on. I flat out told her that she had agreed to a certain price, and that if she had changed her mind, she was free to leave without the stroller and I would be happy to sell it to one of the other 7 people who were interested. She did pay my price, but OY, it was a headache.
I just don't get people like that