My grandfather was a traveling salesman, and when I explain this to our kids, they have to imagine a world where such people existed, In our current time, going door to door trying to sell things would absolutely just get the police called on you. And for expected reason! I don’t want random strangers knocking on our door any more than the next guy. But back in the day, grandpa supported his family this way, going door to door, street to street, neighborhood to town, selling his wares plus collecting a commission, and for multiple years, when I was too young to remember, he sold air filters. In around 1965, air conditioners were becoming particularly widespread, almost every residential area had one. A lot of people were so baffled with air conditioning that they were not even aware of air filters needing to be replaced on a usual basis, but my grandfather did pretty well selling air filters directly to the consumer, plus in multiple cases showing the customer how to install them. These days you can buy air filters for your air conditioning unit almost anywhere, even our local grocery store keeps them in stock, but back then it was much more of a speciality item. Grandpa retired long before the huge hardware stores such as Home Depot sold AC filters for pennies on the dollar of what he used to sell them. So his timing was good. It’s funny to guess that something as self-explanatory as an air filter for your AC device could go from being so precious to being so common.