There are adding more and more from this kind of dark marketplaces and some are pretty interesting but its depending on what kind of goods, there are also a lot of scammers that is the downside.
Yep they are growing and this is because everyone wants to be more anonymous than back in the days what is good but also can turn into bad things.
They are probably growing in popularity because of the stupid war on drugs bullshit, as more people learn about them they will use them so as not to get caught up in a stupid war that could ruin their lives over a joint.