Those are some great tips above, and I'm gonna put them to use quick.
I'm a fan of estate sales, as grills are often underpriced, and the sellers are often ready to deal. You have to read the ads carefully, though, as grills are often described in strange ways. I chased down a listing for 'Weber gas charcoal grill' at the urging of an eagle-eyed friend, and it turned out to be a black SS Performer with all the parts for 25 bucks. Yes, I was happy that day. I also checked on an estate sale listing for a 'Weber charcoal grill' that turned out to be a sweet '88 red One-Touch Plus. There was no price on it, so I asked and they invited me to make an offer. I kicked the tires a bit and said 'Ten?', which they happily took. Yes, I was happy on that day, too.
I'm no expert by any means, but I can tell you what CL sellers have said they appreciated when I talked with them after the sale. When replying to an ad, always be friendly and gracious, and always leave your phone number (even if they never call you, it shows that you are sincere). Also, if you think their asking price is fair, just offer to pay the full amount. I'm convinced I've scooped kettles (and other CL stuff) from low-ballers and hagglers just because the sellers felt insulted and hustled and wanted to get what they were asking. So of course if you really don't want to pay it, offer less, but if the price seems fair (or just a little high), I think you increase your chances dramatically by saying, "That's a fair price for a good piece, and I'll gladly pay it as long as everything is in good shape".
And then, of course, be ready to get to them quick, as sellers are mostly wanting to quickly unload their stuff. I know that's what I'm after when I sell something on the mighty CL.