Every one will admit that swindling ones fellow beings is a necessary practice; upon it, is based really sound commercial success ___________.

Swindling means depriving someone of possession or money through deception or some act of deceiving. As the statement points out, it is considered a required or necessary thing to make money, or have commercial success. We have to pick the right option by saying what this tactic is based on. We have to pick what refers to taking someones money by deception.
Option (c) is right, because dear is also used to describe something that is expensive. Buying cheap and selling at a high cost is an act of swindling.
Option (a) is incorrect, because if there is an item which you cannot buy back from the one you sold it to, then you might get in trouble. It is also a tactic which companies apply, but this is not what the companies base their commercial success on, or it doesnt describe a policy which companies adopt.
Option (b) is wrong, because of a similar reason to option (a), but option (c) is more appropriate, as it says buy cheap and sell at a very high rate, but option (b) does not quite point to making that huge margin of profit.
Option (d) is wrong, because it means selling whatever you can, irrespective of the cost at which you bought it.