They appear to be running their own map servers that just use OSM data, so that doesn't apply here.
map#.craigslist.org. The leafletjs api makes this easy to do.
Though that answer is over a year and a half old.
Perhaps a commercial OSM partner like MapQuest, MapBox or Cloudmade are providing the tiles?
Looking at that same tile through my proxy (which is backed by the MapQuest map tiles), it is clearly different data.
Here is mapbox, which is also different... http://d.tiles.mapbox.com/v3/examples.map-vyofok3q/15/5242/1...