Ethically, it would be nice if a seller included a matching 
douga with the cel sold, since many cels do come with 
the matching sketch. However, with genga, its an 
entirely different matter. In Japan, to many collectors, 
genga is worth a whole lot more than the actual cel, so 
most genga is sold separately. Lots of online dealers 
like Taro and others sell the genga by itself, since a lot 
of the genga is done by the original key animator or 
character designer. Nowadays, there are some 
prominent dealers (who will remain unnamed) that do 
separate the douga and the cel and sell them 
separately. It's all supply and demand. If a show is 
popular enough, douga will often sell. If you don't like 
the practice of buying douga, do your part and just don't 
buy from that dealer. If douga remains unsold, he'll get 
the message.