When Borden is in prison in The Prestige, he receives an offer to sell his secret. Eventually he does so, but to his suprise the purchaser is
Angier
who destroys the secret and is surprised about it later.
But I wonder if he would have known had he looked. Did Borden's solution reveal the truth about how his trick really worked?
There's no way to know, it's never shown to us.
He could have done so, as he was getting desperate to save his daughter and himself. Angier could've saved his life had he still been obsessed about the secret.
On the other hand, he could also not have given away the real secret like the whole misdirection with his notebook earlier.
Selling a trick implies making the purchaser able to reproduce the trick. Logically, that entails knowing how the trick works.
The only fringe exception here would be complex card/math tricks with easy steps to reproduce but being hard to understand the workings - but that just isn't in scope of what we're discussing. The majority of tricks we see are visual, and generally rely on hiding something (that you very much know exists) from the viewer, e.g. not revealing the collapsible bottom in the bird cage for the disappearing bird trick.
If Borden didn't tell Lord Caldlaw how the trick is achieved, then he arguable didn't sell the trick, he just scammed Lord Caldlaw into buying something that is not what was agreed upon.
Another fringe exception here is Borden coming up with a completely different way to perform the same tricks, and specifically selling the version that's different from the one he performs, but this is again out of scope for what we're discussing.
