In order to post listings that are technically different using the same inventory item, you would just need to check the box next to "Is this an inventory item?" below the description editor for each listing and select the appropriate inventory item. There is not anything in place that would prevent you from selecting the same inventory item for multiple listings.
You don't actually have to associate a master profile with each of your inventory items. However, if you do, when you are creating the item for which the title or other information differs from that in the master profile, you will just have to edit the fields that differ from the master profile after the master profile loads upon selection of the inventory item.
I hope this helps. If you have any other questions about your account, please feel free to contact our customer support team using the form on the following page of our site: http://www.auctiva.com/help/requesthelp.aspx