Try using following code:
...
<ListBox.Resources>
<Style TargetType="{x:Type ListBoxItem}">
<Setter Property="LayoutTransform">
<Setter.Value>
<ScaleTransform x:Name="scaleTransform" />
</Setter.Value>
</Setter>
<Style.Triggers>
<Trigger Property="IsSelected" Value="True">
<Trigger.EnterActions>
<BeginStoryboard>
<Storyboard>
<DoubleAnimation Storyboard.TargetProperty="LayoutTransform.ScaleX" To="1.2" Duration="0:0:.3" AutoReverse="True"/>
</Storyboard>
</BeginStoryboard>
</Trigger.EnterActions>
</Trigger>
</Style.Triggers>
</Style>
</ListBox.Resources>
...