DataModel:
class Reseller
{
    public Dictionary<int, Car> Cars;
    ...
}

class Car
{
    public string Model { get; set; }
    public string Year { get; set; }
    ...
}


<!--Template1 contains a Dictionary<string, Reseller>-->
<!-- Reseller has another dictionary of Cars like Dictionary<int, Car>-->
<DataTemplate x:Key="Template1">
    <StackPanel Orientation="Horizontal">
        <GroupBox Header="{Binding Key}">
            <ListView x:Name="Internal_Template2
                      ItemTemplate="{StaticResource Template2}"/>
        </GroupBox>
    </StackPanel>
</DataTemplate>


<DataTemplate x:Key="Template2">
    <Grid>
        <TextBlock Text="{Binding Template1.Value.Cars.Model}"/>
        <TextBlock Text="{Binding Template1.Value.Cars.Year}"/>
    </Grid>
</DataTemplate>