The idiom to sell like hotcakes comes from the United States where the term hotcake was coined in the nineteenth century. It is simply a synonym for a pancake. For something to sell like a hotcake it must be bought in mass quantities with little effort on the part of the seller, usually so much so that it is hard to keep up with demand.
Sell may be conjugated to all its forms in this idiom. And in some instances, the verb can be changed and the meaning is kept. Since the term hotcakes has decreased in popularity, its use almost always carries the connotation of this idiom.
The origin of this idiom is largely undocumented and has led most to believe it comes simply from pancakes being a popular item for sale at fairs and festivals.