Niche question

I have been trying to sell on ebay for a few months now and when I started I foolishly signed up for a course, an expensive one, where I had coaches helping me get the process started. I was told that niches like jewelry, electronics, baby clothes, and supplements were so over saturated that I would have to cut prices to the point where I really wouldn't have that much of a profit margin with them. I have been trying to sell home decor things like wall decor, water fountains and even "collectibles" like batman cookie jars. I just made this account after a chance click of a supplement forum because that is what I am most interested in. My question is are these niches that I mentioned earlier really over saturated? There are some posts on here that I have been looking at that claim to have dropshipping/wholesales with these niches and are also claiming to, if you sell enough, have what looks like pretty decent profits per month. Any advice is appreciated.

I dont think any niche is really saturated, you just need to find a micro niche in the niche

Meaning if you will try to sell brand new iphones you will have to compete with apple or something
But if you go to a sub niche of iphone accessories, you can sell cases or whatever

Because the iphone niche is crazy,maybe you will have to "go deeper " within the niche levels which means

Iphones > accessories > iphones cases > a more specific form of iphone cases

that way you will have less competition

hope that helped

Thanks that does help a lot my so called "coaches" were never really interested in helping pick a micro niche. I'll look into that.