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.