A slightly longer answer is that these are fake silver bars, made of a base metal (such as copper or lead), and (presumably) have silver electroplated on them. We estimate that there is less than $.05 worth of silver on each one.
'Mills Silver' is a term apparently made up by the CMC Mint (selling on auction sites as ameropaintball). 'Mil' is a term used to indicate a thickness of silver/gold plating. Until February, 2010 we had not heard of the term 'mills silver'. At that time, though, the CMC Mint started mass producing many different bars of a base metal and labelling them ".100 MILLS .999 FINE SILVER - 1 TROY OZ" or ".100 Mills.999 Fine Silver - 1 Troy Oz Ounce" or "1 OZ .100 MILLS.999 FINE SILVER" or "100 MILLS FINE SILVER - 1 OZ TROY" or "100 mills .999 Fine Silver - 1 Oz Troy" or "1 Troy oz - 100 mills - 999 Silver" (and probably some other variations).
The term 'mil' when electroplating silver refers to 1/1000". The items marked '100 mills', if referring to 'mils', would be 1/10" thick on each side -- yet a standard real silver bar is 91 mils thick (.091"). Clearly, these are not electroplated with 100 mils of silver, or they would really contain about 2 ounces of silver, and the company would quickly go out of business. Therefore, the ones marked '.100 mills' probably mean '.100 mils', or 1/10,000". That is 1/910th the thickness of a standard silver bar, and multiplied by 2 (as all sides should have the electroplate on them) is 1/455th the thickness of a standard silver bar. That's about 0.22% silver, or 22 parts silver per 978 parts of lead/copper/whatever. Or a value of less than $.05 even with silver at $20/ounce.
The term 'mills' has a legal meaning in the United States; it refers to 1/10 cent. As such, these bars may be considered counterfeit coins.
Of course, if you find one of these stunningly beautiful, and don't mind the legal risks of buying and owning one, and don't mind that you won't be able to sell it later, go ahead and buy one.