Directories have various features in listing, often depend upon the price paid for inclusion:
Free submission
Reciprocal link
Paid submission
Each ad posting in a free classifieds directory becomes a distinct webpage and this automatically provides a good deep link for an advertiser.
Why Do You Need To Ping a Website?
A ping is simply a notification to a server that content has been updated. These ping servers are used by other services (including what we're interested in, search engines).
