Generally speaking, these cupboards were built into a corner(s) of the room in period which is why they are almost always painted. The ones that are period are almost always made from pine/poplar as you see many come to market stripped of paint. I dont think this is any thing to worry about though as they are salvaged from old houses that are being torn down/remodeled and in any case this cupboard on ebay is a repro. Probably the most famous period architectural corner cupboards in America were made on the Eastern Shore of VA and are quite spectacular although very pricy, even if un-restored.
The ad says this cupboard was made in France in the 1st half of the 20th century so its most likely container goods shipped here from europe that did not sell at auction for one reason or another.
The shell looks like metal to me.
I am going with snowflake on this one, it does look like metal but hey, whats wrong with that?