So basically there is a set of doujin that are sold at events like comiket or the Comic (Star) events that differ from the published ones. I've noticed sometimes its just a preview of a coming comic but other times its the full issue.
I don't know how to explain it better, but a lot of times the event sold ones don't have a color cover. What I'm wondering is, what is the purpose of these? Why are they sold for more money on places like Mandarake? Sometimes its two doujin for 1,000 yen each that make up a collection with a color cover sold for only 600 yen. Is the quality better on the former? Is it because they are rarer?