There's no right answer, just how each individual wants to handle it. I for one don't like offers, low ball or not. Frankly, I find it rude if the first thing that comes out of your mouth is "Would you take....", which is vastly different from "Hi, my name is Chuck and I live about 5 minutes away. If you don't sell it for your asking price, I'd be willing to go $400". Even more rude are those dudes that offer less, you say OK, they come and look and pass.
If I'm selling something and asking for $500 and within a short time frame I get a lot of inquires, I know I'll sell it. If it's a stupid offer like "Would you take $100 CASH!!!!????" I'll usually ignore.