פירוש על עבודה זרה 115:5

The market inspector was not drinking wine for his own benefit. He was testing it to see what it was, perhaps its quality. Nevertheless, the rabbis prohibited not only the wine that he actually touched, but the entire cask that he threw the wine back into. This is a proof against Rava.

Rava tries to answer that it is prohibited only to drink the wine, but not to sell it. The problem is that the second half of this baraita uses the word “it may be sold” to rule that while it is prohibited to drink the wine, it is not prohibited to sell the wine. Thus, when the first half states “it is prohibited” the implication is that is prohibited to even sell the wine.
As far as the difference between the first half of the baraita and the second half, the rule seems to be that if the non-Jew knew it was wine, we must be concerned that he libated it, even if this is very unlikely. But if he thought it was oil, then we can be sure that he did not libate it, and thus the wine can be sold. Note that the rule is still strict with regard to drinking the wine. Even though the non-Jew touched it not even knowing if its wine, it is still prohibited.