My only thought was I should not need to spend hours to learn another states laws to sell a gun. If they were at least SIMILAR, like the drinking age per se, The chances of unknowingly breaking a STATE law would be a whole lot less.
This statement indicates a lack of understanding of interstate firearms transfers. Your first sentence implies that your concern is with interstate firearms transfers, ie you a resident of one state wish to sell or transfer to a second party of another state. These transactions while affected by various state laws, are also governed by federal law. As a side effect, if you follow federal law, you will find it hard, although not impossible(certain situations of a long gun transfer could go awry of state law), to break state law. Interstate firearms transfers, with few exceptions, must go through a FFL. One law to know that will protect you from accidentally breaking the law a majority of the time. It's really not that complicated.
However, this thread is about universal background checks forced by the federal government, which primarily affects private party transfers in one state, so your concern of "needing to know multiple states laws" is moot. Which brings us back to...
And the more important point which you have missed is that the federal government has limited authority when it comes to making "universal" laws. It is important to understand that, particularly when the proposed "universal" laws apply to a fundamental right to which the federal government is expressly constrained from restricting. In this specific case, please cite the authority with which the federal government can regulate an intrastate transaction between two private persons.