compare this:
But there were false prophets also among the people, even as there shall be false teachers among you: which privily shall bring in damnable heresies, even denying the Lord that hath bought them, and bring upon themselves swift damnation.
And many shall follow their destructions, by whom the way of truth shall be evil spoken of.
And through covetousness shall they with feigned words make merchandise of you, whose condemnation long since resteth not, and their destruction slumbereth not.
(2 Peter 2:1-3)
Peter says there will be "many" with feigned words making merchandise of you.
Paul similarly says they are not like "many" are, making merchandise out of the gospel.
Thayer's interjected opinion is that it's best to mistranslate the word as 'corrupt' but as Peter describes, these false teachers use 'feigned' words in order to make merchandise. to adulterate in order for gain, which is, to peddle dishonestly: that is the exact meaning of the idiomatic sense of the literal word, and the context of 2 Corinthians 1:17 makes it quite clear that the merchandising going on is of the dishonest sort.
Thayer's argues to compare this verse:
Therefore, seeing that we have this ministry, as we have received mercy, we faint not:
But have cast from us the cloaks of shame, and walk not in craftiness, neither handle we the word of God deceitfully: but in declaration of the truth we approve ourselves to every man’s conscience in the sight of God.
(2 Corinthians 4:1-2)
here Paul says they do not walk in craftiness - which is, they do not seek their own gain, i.e. they don't peddle. continuing, in verse 5 he says they are our servants for the sake of the gospel. this supports the original English translation of 2:17 just as much, if not more than king James's, especially when taking into account that Peter expresses the same sentiment about the same people in terms of salesmanship.
KJV has a weak argument here, IMO, and the argument is at best based on not literally translating but instead taking liberty with the text in order to project a particular subjective interpretation. even that subjective interpretation is not about 'changing' the goods being peddled, as it were, but about misrepresenting them, their use and value, so it's not very convincing to me in any case that it's in any way 'better' to mistranslate it as 'corrupt'