I think that Randists (or Objectivists, if you prefer) miss a fine point...
Obviously, yes, by definition it is in everyone's self-interest to be self-interested. Thus researching mechanics makes sense. That's basically all you are saying- a circular argument. A tautology.
Unfortunately Rand tries to build a system of ETHICS around self-interest, and it doesn't work. Or, at least, it results in a world in which I wouldn't want to live. (See my point about laissez-faire capitalism, above. Under such a system, if you start poor and downtrodden you will almost certainly die poor and downtrodden, and it doesn't matter how self-interested you are.)
What you're saying seems to imply that if I take my car to a mechanic without thoroughly researching him first and he cheats me... I'M the unethical one. This is ludicrous. I may have been too busy or something, and made an ill-informed choice, but I certainly wasn't unethical. Under Rand's system of "ethics" the mechanic doesn't deserve to be punished- he did nothing unethical if he weighed the pros and cons and decided that cheating me was in his self-interest.
And obviously many do in fact decide that cheating people pays- particularly in a large city where there are always going to be enough people floating around who on that particular day got too busy to spend hours researching a mechanic. Saying "Yes, but if EVERYONE ALWAYS acted in their own self-interest it would work" is ludicrous. One cannot always be that diligent. (Even an Objectivist.) And if one were, that's basically paranoid dementia. I don't want to live like that.
I maintain that the mechanic did something unethical, and in fact criminal, and deserves to be punished.
Constantly being out for oneself basically makes one equivalent to an animal. And, heck, all creatures have to eat so I don't think you'll be actually faulted for being self-interested. But being honest and displaying some integrity and compassion makes you ethical and admirable.
I'm much more of a Utilitarian than an Objectivist. I think John Stuart Mill was on to something. Read that last sentence carefully- I don't think he had ALL of the answers. There were obviously bugs to be worked out under his system, and on a few points for entirely practical reasons he DOES get a little Objectivist, but it beats the heck out of Kant or Nietszche. This is in stark contrast to the real Objectivists, who treat everything Rand ever wrote or said as if it were holy scripture. In fact, Objectivism has been compared to a cult, which is starkly ironic for a bunch of avowed atheists.
But Rand got that much right- it's about happiness.
Not to mention that this point:
"For example, it is never in one's long-term self-interest to cheat someone, financially or otherwise. For a couple of reasons, not the least of which is a sense of guilt about taking advantage of someone else. If one has a well-developed conscience, living with the knowledge that one cheated another party has to be considered a “cost” borne by the cheater."
is a self-defeating one for Objectivists, because it presupposes another ethical system! If one were a pure Objectivist and decided that it was in one's self-interest to cheat someone then why would one feel guilty over doing so? After all, you had done the ethical thing, under Objectivism. The only reason to feel guilt (to have "a well-developed conscience") over cheating someone is if you follow a system of ethics OTHER than Objectivism.
That, alone, kind of blows Objectivism out of the water. (And if you leave such pathetic quibbles out of Objectivism it results in a world of self-serving pitiless monsters.) Thus long-term self-interest MIGHT be considered a decent rule-of-thumb under any other ethical system, but it sure as heck can't be the basis for its own entire ethical system. Something else is.
That said, I'll +1 your entire second paragraph.