What's interesting about conceptual art is the concept. The idea of duct taping a banana to the wall and then selling it is puzzling and brings up all the questions that you pose. Why do it in the first place? Because it creates this compelling set of questions, and also because, since art is usually a physical object, there is also something appealing about the object itself: a humble banana humbly duct taped to a wall that is being elevated to the conceptual status of art. It certainly has interested you enough that you would ask all these questions. You bring up a lot of other pieces of conceptual art that you have thought about, so it seems to me that you are getting it.
In this case the only skill of sticking the banana to the wall is in conceptualizing the idea, not in the actual act of applying the duct tape, but in much of modern art there is great skill in applying paint in a particular way, for instance, as in color field paintings or abstract expressionism.
As far as the actual banana, the buyer bought the rights to duct tape a banana to the wall and to call it the work of that particular artist. The banana itself can be replaced as often as the buyer wishes to.