Not sure if I should have had to put those skins in the skins/default/en folder like I did or in skins/custom_skin/default/en. It works fine in the former:
https://docs.google.com/file/d/0B46HNJGXAQ6gb2MxTWJrSlNWUlU/
Puts the Our Price label and the Market Price just in the grid, and nowhere else.
I experimented by creating the same two templates in the custom skin directory:
skins\custom_skin\default\en\items_list\product\gr id.product.market-price.tpl
and:
skins\custom_skin\default\en\items_list\product\gr id.product.our-price.tpl
And nothing happened. I think the custom skin can only overwrite templates, not create new ones.
Since the templates are created in the default/en category, I can now override them in the custom_skin directory with this path:
skins\custom_skin\default\en\modules\XC\CustomSkin \items_list\product\grid.product.market-price.tpl
and:
skins\custom_skin\default\en\modules\XC\CustomSkin \items_list\product\grid.product.our-price.tpl