No, I don't have any plugin installed and will code the functionality myself. Yes, will have to write the code to write the posts data from the text [DELIMITED] or XML file as may be appropriate. I have done almost a similar project for WP 2.8+ using the table prefix for the different domains.
Now have to do almost the same for a 3.1+ installation with 180+ sub-domains and nothing like table prefix sort of thing available - but blogid. I am not sure how the the 'Domain Mapping' plugin arranges the different domains under a main site.
One other option could be to place the listings in a single main site and share it across the domains? any comments on this will be appreciated.