The Target property in ChangePropertyAction can only select named elements. You can define a FindAncestor binding, then use "Name" as the binding Path, in case the specified control type has a name.

I think we need to extend the Target property editor to support objects too, not only names.

Indeed. I can make it work writing it like this:

      <b:ChangePropertyAction PropertyName="IsSelected" Value="True" TargetObject="{Binding RelativeSource={RelativeSource AncestorType={x:Type ListBoxItem}}}"/>

Instead using the Studio I'm getting this:

      <b:ChangePropertyAction TargetName="{Binding Path=IsSelected, RelativeSource={RelativeSource AncestorType={x:Type ListBoxItem}}}" Value="True"/>