Well we all make mistakes in life. Sometimes we think that we are doing the right thing, and from our perspective it all looks good. Well, then we stand back and look at it from someone elses perspective and maybe it doesn't look the same.
If the buyer was really serious I think that he would have been willing to discuss the situation. Because until he handed over the check, which I will address in a moment, he hasn't purchased a car, a beautiful NSX. Now to the check, something that no one has thought of, was there really one? Did the guy bother to fax you a copy of the check so you could verify funds with the bank? I think that would have been an act of good faith on his part. There are people who may try to give you a bad check and run off with your NSX and signed title. You would not have known for at least two days. The dealer would have checked out the check. No pun intended. You are better off having your car and not having a worthless fake cashiers check. Hope that makes you feel better.
Next time better communications as far in advance, with full disclosure and advantages explained to include $$$$$$. I have learned a lot from being married to my wife for 24 years. It hasn't been easy on me.
Brad