установить Datacontext сетки с помощью SelectedItem of Listbox, а затем выполнить обычное связывание, как показано ниже
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="3*"></ColumnDefinition>
<ColumnDefinition Width="5*"></ColumnDefinition>
</Grid.ColumnDefinitions>
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"></RowDefinition>
<RowDefinition Height="*"></RowDefinition>
</Grid.RowDefinitions>
<Button Margin="7,7,7,0" Padding="2" Click="cmdGetProducts_Click">Get Products</Button>
<ListBox Grid.Row="1" Margin="7,3,7,10" Name="lstProducts" HorizontalContentAlignment="Stretch" SnapsToDevicePixels="True">
<ListBox.ItemContainerStyle>
<Style>
<Setter Property="Control.Padding" Value="0"></Setter>
<Style.Triggers>
<Trigger Property="ListBoxItem.IsSelected" Value="True">
<Setter Property="ListBoxItem.Background" Value="DarkRed" />
</Trigger>
</Style.Triggers>
</Style>
</ListBox.ItemContainerStyle>
<ListBox.ItemTemplate>
<DataTemplate>
<Grid Margin="0" Background="White">
<Border Margin="5" BorderThickness="1" BorderBrush="SteelBlue"
CornerRadius="4">
<Grid Margin="3">
<Grid.RowDefinitions>
<RowDefinition></RowDefinition>
<RowDefinition></RowDefinition>
</Grid.RowDefinitions>
<TextBlock FontWeight="Bold" Text="{Binding Path=ModelNumber}"></TextBlock>
<TextBlock Grid.Row="1" Text="{Binding Path=ModelName}"></TextBlock>
</Grid>
</Border>
</Grid>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
</Grid>
<GridSplitter Grid.Column="1" HorizontalAlignment="Left" VerticalAlignment="Stretch"
Width="5"></GridSplitter>
<Border Grid.Column="1" Padding="7" Margin="7" Background="LightSteelBlue">
<Grid DataContext="{Binding ElementName=lstProducts, Path=SelectedItem}" >
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto"></ColumnDefinition>
<ColumnDefinition></ColumnDefinition>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"></RowDefinition>
<RowDefinition Height="Auto"></RowDefinition>
<RowDefinition Height="Auto"></RowDefinition>
<RowDefinition Height="Auto"></RowDefinition>
<RowDefinition Height="*"></RowDefinition>
</Grid.RowDefinitions>
<TextBlock Margin="7">Model Number:</TextBlock>
<TextBox Margin="5" Grid.Column="1" Text="{Binding Path=ModelNumber}"></TextBox>
<TextBlock Margin="7" Grid.Row="1">Model Name:</TextBlock>
<TextBox Margin="5" Grid.Row="1" Grid.Column="1" Text="{Binding Path=ModelName}"></TextBox>
<TextBlock Margin="7" Grid.Row="2">Unit Cost:</TextBlock>
<TextBox Margin="5" Grid.Row="2" Grid.Column="1" Text="{Binding Path=UnitCost}"></TextBox>
<TextBlock Margin="7,7,7,0" Grid.Row="3">Description:</TextBlock>
<TextBox Margin="7" Grid.Row="4" Grid.Column="0" Grid.ColumnSpan="2"
TextWrapping="Wrap" VerticalScrollBarVisibility="Visible" Text="{Binding Path=Description}"></TextBox>
</Grid>
</Border>