I wouldn't get my "undies in a bunch" over an offer that I'd consider "low ball", when I've gotten them, it's usually someone contacting me through the email feature, so they are not really "committed" to making that offer. I usually respond with a price I'd consider and leave it at that.
Speaking of using the email feature, I'd not at all consider an email contact as an "offer", I'd insist that the potential buyer make a formal offer on the site. One time the potential buyer asked for my cell number, as he said he had some questions, I gave it to him and what he really wanted to do was negotiate, we came to an agreement on the price, but when he arrived at my home for the pickup, he said he thought about it during the drive and said he'd only offer $600 less than we'd agreed upon. "No Thanks". After that experience, I've insisted on the buyer making an offer on the site.