I have sold items on Creative Market and Envato for some time (I am no longer selling becase I sold the copyrights to the files). The market is pretty saturated with goods and a lot of your designs will get rejected on Envato based on this. I haven't had any of my designs rejected on Creative Market. On CM, I've seen they are screening more rigorously when accepting a new shop, not on individual items.

On Envato I've seen people actually pay for advertising on other websites or social networks in order to promote their designs, so I'm not sure how passive it is any more.

I was able to earn a good amount every month for some time, but that was a few years ago.

If you know you can design a large number of files each month you have another option, Freepik. They pay $5 / 1000 downloads. If you have a lot of files you can earn some money this way, also. Hope this helps.

I've been selling a non-design product on Gumroad - so I may not be totally qualified to chime in on selling design products on it, but in my experience I wouldn't say it's not too saturated.

That being said, you're going to need to do some additional work other than just putting it up there and hoping people check it out. Very few people will find it directly through Gumroad. Most people (probably over 90%) who do land up purchasing the product I sell get to Gumroad through my own website.

Regardless, it's a worthwhile and fun pursuit that can teach you a little more about digital marketing, running a website, etc. I'd highly recommend at least giving it a shot :-)