^ I would ask the seller in this way: If it is still available, I will pick it up in person with cash. When and where can I meet with you?
That way you sound more sincere to start with. Concise and productive communications save time and make it easy for both parties.
As an example, after reading a well-described for sale thread from an established Headfi member, I often start with "I will take it. Please let me know your paypal".
Alternatively, I may say "I am highly interested. Please help me to confirm the following items: ......."
People on Craigslist are rarely serious though; some may tell you to meet at XYZ and never shows up. A forum in your university or eBay may be a better place.
Yeah I guess I should try that as well and not go with the casual craigslist message, but I didn't exactly say anything out of the ordinary but have gotten quite a silly responce.