In the past local searches (product + locale) would occasionally return an “inventory” onebox if Google felt that it was more relevant than a Local 10 Pack. An example is the search “Real Estate + Locale” which provides a Google Base search box to explore available inventory.
Recently on the searches for “used cars + locale” Google has started including both the Local 10 Pack AND the Inventory Search Box.
Over time, Google has been increasing the granularity of their results as they have adequate data to provide meaningful results. Providing a list of currently available used autos in an area is certainly part of this trend.Google Maps - Multiple OneBoxes on Used Car searches by Mike Blumenthal