Why would using an upgrade certificate bought on E-bay be considered stealing from the airline? If the airline has confirmed space for the upgrade, does it matter who uses it?

Is there any difference if the person selling the upgrage was using a mileage based upgrade certificate from miles purchased buying kellog's products?

I realize two wrongs do not equal a right, but it seems the airlines constantly get away with all sorts of unethical behaviour and break state laws daily.