I'm not an expert cold-caller by any means, but I did once buy a red JJ out of the back of a pickup truck in the grocery store parking lot. It was a little weird, but I just smiled and said 'hello' and then complimented them on the grill. I didn't tell them how rare it was or anything, but just said I liked red Webers and they had a great specimen. Within a couple of minutes the guy was trying to sell it to me. I offered him 20 and he countered with much more and we settled on 45. Both of us very happy with the transaction.
All to say, don't be in any huge hurry to close the deal. Try expressing some interest/admiration, and you might be surprised at what comes your way. Or like Duke said, they might not want to sell it right then, but would sell it to you later. Try to come alongside them as a fellow Weber fan, and if they're not actually a Weber fan, they might be interested in making a deal. Either way, you've hopefully made a friend.
Oh, and if they are a fan, tell them about WKC! I mentioned it to this dude who sold me my red SS, and he ended up becoming PeasPurple! Lol.