What designates a restaurant for sale as a diner for sale? It could be the wide range of food offered (though specializing in American cuisine is a given). It could be the casual atmosphere or counter service. It could also be related to the hours. Most restaurants in America that classify themselves as a diner for sale work late hours or even around the clock.
Diners have been part of the American restaurant history since the 1920's when many of these were actually prefabricated offsite in factories (like mobile homes or today's tiny house craze). They were then taken to the site and dropped, often by rail car or truck which meant most of these restaurants were long and narrow.
Quick food fast -- these diners for sale are early precursors to fast food. The standard food consisted of all American classics like hamburgers, fries, club sandwiches and other simple and inexpensive foods including breakfast nearly 24 hours a day. It is common to see milkshakes prepared onsite along with pie under glass and other sweet dessert offerings. The restaurant brokers still see diners for sale that originate from the 1920 or 1940's but many more examples of this cuisine operate today in nice modern buildings.
Wikipedia reports that the first diner in America was created in 1872 by Walter Scott, who sold food out of a horse-pulled wagon to employees of the Providence Journal in Rhode Island. They say that "Scott's diner can be considered the first diner with "walk up" service, as it had windows on each side of the wagon."
It wasn't long until lunch wagons were produced commercially. In 1887, Thomas Buckley became known for his "White House Cafe" wagons. The concept was patented in 1893 for a concept he called the "Night-Lunch Wagon."
Today's diners for sale have many things in common with the history of the American restaurant scene including nearly 24 hours service, inexpensive food targeted to American taste and lots of hot coffee.