Objectivism requires, in a nutshell, that you do not attempt to gain values through dishonesty. This means more than simply ensuring that what you say isn't technically a lie; it requires that you endeavor to appeal to others' reason and intelligence rather than their stupidity and gullibility. In both of your examples cited above, the person is clearly behaving dishonestly, and in both cases it comes in the same form. The person is failing to disclose a fact that they know will be material to the decision of the person that they are tricking. In your original example, the guy clearly knows that he's going to leave this girl as soon as he sleeps with her, and he also knows that she wouldn't sleep with him if she knows this. He's deceiving her by withholding this fact and pretending that he has the intention of dating her. Similarly, the fact that some part will soon go out at great cost is a fact that is material to the buyer's decision to buy. Withholding it is fraud, and clearly dishonest.
Objectivism holds that this method for gaining values will not serve your life and happiness in the long term. Relying on dishonesty to gain values requires that you seek out the dumbest and most gullible people to deal with, rather than the most intelligent and perceptive. It institutionalizes a fear of certain facts, namely the facts that will expose your lies, rather than encouraging an attitude of unreservedly confronting all facts of reality, which is the policy that one needs in order to be successful over the long term. Furthermore, relationships founded on dishonesty cannot become the kind of deep relationships that are integral to one's happiness, where another person truly sees and understands you. No short-term gains of one-night stands or car sales are worth this kind of life.