One of the fascinating things about Craigslist is its informal post-sale sanctioning system. That is, if you don’t take down your post after you sold the item, you get an increasingly annoying stream of emails from people asking questions about the item. This continues, of course, until you actually remove the post offering the item you sold. It is a great example of virtual community gardening.
Because of this sanctioning system, we can make a reasonable inference that items that have been taken off of Craigslist have been sold. The items that have short lifespans on Craigslist are desirable – they are a good value, priced properly – and those with long lifespans are either unwanted or improperly priced. I’ve recently been in the market for a used car (cough, a minivan), so I’ve been collecting information about the cars offered on Craigslist and their lifespans on the service. By looking at prices and lifespans (and a few other variables), can we automatically identify cars that offer the greatest value?
What follows are some charts from a simple survival analysis of the last 30 days of Honda Odyssey sales on Craigslist in Raleigh/Durham. The de-duped dataset includes 55 cars (out of about 130 posts). Before you read much into the data, many of the variables I explored (mileage, model year, etc.) weren’t significant predictors of “hazard” (that is, sale). If you were able to get this data on a larger scale, it does seem likely you’d be able to identify patterns of value. That said, there is a lot of randomness is a car’s quality once it has been driven, so the value of such a model-based approach would only be in prioritizing potentially under-priced cars.
n.b.: You could also do this sort analysis on want-ads. Want-ads have a great sanctioning system, as it is pointess to pay for an ad after you’ve sold your car.
p.s.: Perhaps what is charming about Craigslist is that there isn’t any meaningful historical data. This likely generates more variability in price, leading to the perception that you can find great deals (which you can!).
neat-o. don’t people also remove postings from craigslist so that they can re-post at the top of the page? i suppose that is where the de-duping comes in?
Yeah. As it happens I’ve got a cache of those re-posts. They are interesting to watch – the price goes down and they get more desperate. They do need to be de-duped for the analysis, though.