Is mine to be worth nothing to me?
There is no deception in the affair.

A jeweller's business is not like a horse-dealer's.

The stone is as God made it, and the day is as God made it, only my knowledge enables me to use both to better purpose than my neighbour can." "Then a man's knowledge is for himself alone--for his own behoof exclusively--not for the common advantage of himself and his neighbour ?" said Cosmo.
"Mine is so far for my neighbour, that I never offer him a stone that is not all I say it is.

He gets the advantage of his knowledge, let us say, in selling me wine, which he understands to fit my taste with; and I get the advantage of my knowledge in selling him the ring that pleases him.

Both are satisfied.