You can shear a sheep many times, but you can skin it only once.
If you go in for the kill, and really take advantage of someone in a transaction (i.e, skin the sheep), that will likely signal the end of any (positive) transactions with that person, unless you have a monopoly or other such power. Even if they don't realize what you did at the time, eventually they will, and they will resent it.But by just shearing the sheep, you are creating a sustainable profit / relationship with someone, that will have the potential to be repeat business.
This works on the customer side, as well - are you the sort of person that grinds away at every contract and transaction, so that the other side makes (almost) nothing? Well, you are walking the line of skinning that sheep...