I've found myself reluctant in certain situations to 1) count the bills to make sure I got all of my money and/or 2) to inspect the bills to make sure they are not counterfeit. I think the other person will think that I don't trust them. I've seen the same thing from other American's from whom I've purchased items. They never count or inspect the bills (at least while I'm there picking up the merchandise).
If one is buying or selling here a bill-check marker might be a good investment.
I always say something like "I apologize, but I need to count and inspect each bill you are giving me. Unfortunately, I have learned through experience and observation that here in Argentina, this type of precaution is absolutely necessary to prevent mistakes and fraud." Then I apologize again that I am counting slow and double checking everything. In my experience, no one has been offended by that yet, and they usually nod in agreement and appreciate the fact that you are being cautious.
I also have both a bill-check marker and a black light bill checker.