To begin with, much of what is available in health and beauty departments is not soap.
They are chemically derived detergent bars. Try this: take any bar of grocery store "soap", turn it over and enter the ingredients into a search engine. You'll find petroleums, a host of chemicals actually designed for something other then human use, and most likely a couple of things cautioned against by the FDA!
What you probably won't find in that grocery store beauty bar is the glycerin. Glycerin is a naturally occuring by-product of soaponification (becoming soap). Large producers frequently strip glycerin from soap for use in other products. That translucent bar of glycerin soap was made from what should have been left in the other bar of soap! Glycerin retains moisture readily and that's great for your skin! Soaps made by small manufactureres like The Feathered Nest, retain their natural glycerin. Yeah skin!!
So buy natural soap for what's NOT in it and also for what IS in it.