I have always found Fiery and Flash harder to come by, but the last time I saw some of them selling Smokey was weirdly the expensive one. But maybe I just missed him being htf because I had him as a kid so never looked for him. I just know it took me longer to get Fiery and Flash than the rest.
I would also separate them because there will be a lot of people looking for one or two dragons rather than the whole set IMO. Though I get the whole that's extra hassle aspect of it. I wish you luck with your sale