This is essential information for all bullion buyers. And this is why you should always avoid buying silver that is less than 999. People do not desire impure silver and dealers do not want impure silver because it is more expensive to refine to pure silver. Dealers buy Sterling at 80% (or less) of spot. Dealers buy 40% halves at 75-80% of spot Dealers buy War Nickels at even less. However, this does not apply to 90% US junk, as no one really desires to refine that when they can keep it in its easily recognizable form. Thus, when buying silver, buy 999 bullion and US junk.
Excellent summary. The only slight change I'd make is to recommend 90% equally with .999 at the beginning. In fact, if the .999 is non-standard (a little-known manufacturer or a home-poured bar), you might have trouble getting full value from a dealer, because they're facing the risk that it's mislabeled.