[Cyclamen]

--> ことを示している
こと is used to nominalize a proposition (here, ......必要である)
を indicates the verb's object (in this case, the nominalized proposition)
示す　means "to point out", "to show", "to explain". It's used here in -ている (continuative form)
That's why I translated this sentence as
"the general rule of fair exchange explains that ......"

--> 教訓なのだろうか
教訓 means "precept", "moral instruction"
なの emphasizes the noun (教訓)
だろう expresses a conjecture
か is a particle used to mark questions

A literal translation of それは教訓なの だろうか would be then "Would that be a lesson?". The subtle meaning of japanese だろう can be translated in many ways, depending on wheter the speaker knows the subject he's talking about ("that would be a precept") , he's making suppositions ("would that be a moral instruction?" / "could that be a precept?"), or is waiting for a listener's response ("that's a moral instruction, right?")

So, literally, translated, your sentence would sound like
"The principle of fair exchange points out that for the sake of obtaining something, something of equal value is necessary. Would that be a lesson?"