The first item in your selector is only used, when the button is pressed AND selected. If you want to use button_sel
when your button is pressed OR selected, you should do something like this:
<selector xmlns:android="http://schemas.android.com/apk/res/android">
<item android:drawable="@drawable/button_sel" android:state_selected="true" />
<item android:drawable="@drawable/button_sel" android:state_pressed="true" />
<item android:drawable="@drawable/button_unsel" />
</selector>
The items are evaluated from top to bottom, the last one is the default. Though I am not sure if state_selected makes sense for buttons.