"Did I, or did I not provide a clip of the show on the article talk page? Despite the information provided on the website, did the show not declare her as the best selling?" 

 The WMA are not the world authority on who is the best selling artist. 

  "Aren't there hundreds of credible sources in the article and on the web to back up the statement?" 

 No there are only two sources that list her sales, the one from her own website, and the one form WMA, however there are many more sources and more credible sources that say Madonna is including, Guiness and Forbes along with countless other magazines. 

     "Now that I've gotten that out of my system, let me see if I can explain it more clearly. The major issue we're dealing with here is that of "attribution". Here on Wikipedia, when we make a claim, we cite sources to support that claim. Answer me this question: Do the sources not say that Dion is the best selling? Yes they do. The article does not state that Dion is truthfully and undoubtedly the best-selling. It said, according to these sources, she is the best selling, which is accurate; the sources actually stated this. Whether or not sales add up is not up to us to decide (there is a policy that says No original research). What matters is what the sources say." 

 It says no where in the article that "according to these sources she is the best selling..."  
 BTW no I did not notice the video you posted until you said something, you do not need to be rude or condescending to me.