Hi!

I've been thinking about starting to create and sell clothing packs specific to certain eras/locations. Like Formal Office Pack or Medieval Village Pack but was wondering how profitable it would be since as far as I understand it, I can't sell it anywhere else if I do sell it here. Hope I don't come of as greedy but I really like the software and would be glad to add to it's value but it would be nice to get some insight in how much people are earning from their assets!

I believe you are only partially right about not being able to sell content elsewhere...

As far as I know, Reallusion does not have any exclusion clause which prevents your own content form being sold on other sites. Reallusion only prohibits the use of their content, including but not limited to character meshes, from being sold on other sites. This means if your clothing was generated from an RL mesh and still contains a good portion of the RL mesh (such as skin tight t-shirts, jumpsuits, etc) then the content cannot be resold on other sites. However, if your content is only sized for the proportions of RL characters but does not actually include any RL meshes, then you can resell the content on other sites.
Since different software characters (e.g. Reallusion characters vs DAZ Studio characters) have slightly different proportions, if you decide to resell your period clothing on a different site you will probably need to act like a tailor and somewhat re-adjust the clothing for the target character but there should not be anything preventing you from reselling you content in the Reallusion Marketplace and other sites.

I am just a hobby content developer who is definitely not making a living from his Reallusion Marketplace store but my cheaper-alternatives clothing (usually less detailed than the professional packs but at a lower cost) is selling fairly well. A lot of my clothing is intended as bases (similar to the Plain White Bases that come with Character Creator) but are provided with a sample texture. I then generate a bunch of different textures/styles using the base which is derived content from the base. This way if the customer wants to buy more of these textures/styles they don't need to buy the base content multiple times. They but the base once (usually the higher cost) and then they can buy as many of the  different textures/styles that they are interested in (each costing a low price) and use them as long as they bought the base. This way I am not price gouging the customer each time I create a slightly different material/style using the same clothing mesh base.  

"We often compare ourselves to the U.S. and often they come out the best, but they only have the right to bear arms while we have the right to bare breasts"
Bowser and Blue, Busting The Breast