Cicero takes up the deliberation of when the apparently useful conflicts with the honorable. He tells the story of Roman consul Quintus Scaevola, as follows:

Remarkably enough, when Quintus Scaevola, son of Publius, had stipulated that the price for a farm which he was buying should be declared once and for all, and the vender complied, Scaevola then said that he valued it higher, and added 100,000 sesterces to the sum demanded. No one can deny that this was the behavior of an honest man, but people say that it was not the gesture of a wise man; it was like selling for less than he could have got. So this is the pernicious tendency to categorize honest men as different from the wise, a view which led Ennius to say that a wise man's wisdom is vain if he cannot do himself a good turn. This is true enough, if only Ennius and I could agree on what constitutes ‘a good turn'. (3.62)

What does Cicero mean by "if only Ennius and I could agree on what constitutes ‘a good turn'" and how does it fit with Cicero's position in Book 3?