My understanding of the question was that it was in regards to a very small quantity of fish, which the mex government would not be able to track and the US government would not bother to enforce. Obviously if you are selling in large quantities and often,it is illegal cause that is commercil fishing. Regardless of what their laws say about selling sportfish, I cannot imagine a scenario where u sell a few pounds of fish to a buddy and the Mexican or US officials bust you. But hey of u want to follow the letter of the law, then don't sell any fish or game ever, period.
It amazes me how scared everyone is about breaking laws, when the fact is when you drive your car today, you will break the law by going just 1 mph over the speed limit. Is it against the law, yes, will u get busted, no. So in essense it's a matter interpreting the letter of the law vs. The spirit of the law.
And in regards to my posts, keep reading them, perhaps you will learn something.
In your example 1mph over the speed limit could give you a traffic citation, selling sport caught fish is immoral and could land you in prison. They're two completely separate things. I do agree with you though, nothing wrong with giving your buddy some of that fresh catch. Selling it to your buddies for a profit through, completely wrong.