I think final decision should be based on how big is difference in two systems, if difference not big its better to do with one script , different listing types and some coding to show different things for different listing types.

In this case database is one, one admin panel one place for users to manage their listings and so on,

If difference is big it's better to do on different script installations but in this case you will need account synchronization and other things.

My opinion is that better to keep it in one script, it will be much easier to manage.