Want to track new postings on Craigslist for a specific search? Use IFTTT (IfThisThenThat).
People make "recipes" on the site combining triggers like text messages, Google Alerts, FourSquare checkins, and programs a reaction with another service, like Dropbox or Twitter. Learn more
For this example, I want to track the prices of Red Sox tickets on Craigslist. Copy the URL the search generated.
Visit this link and give IFTTT permission to access your Google Drive account.
and paste the copied URL into the appropriate data field.
All new updates to that search will be sent to a spreadsheet in your Google Drive.
You're all set. The data comes in dirty but there are ways to clean it up.
(Hint: Use Text to Columns in Excel to split off the prices listed)