There are indeed a lot of dark marketplaces these days but I do not think that is wrong or something because its not bad and also not good of course.
Dark marketplaces cannot be stopped and there are more currency's used on it so...
As long as there are people who wants to hide themselves underground and doesn't want to have association with legal things. Dark marketplaces are going to exist. Different illegal trades are happening there and as long as there are monetary value involved they are going to remain and no one will ever stopped them for keep on going.
Thats one of the reality on this world which there are really evil people would tend to do illegal things because they could able to generate big money compare on doing legal things thats why we could not stop these activities . Regarding to OP building marketplaces either here or on darknet is still the same.