Product “ownership” questions/branding

I have two conceptually related questions about product branding, but otherwise they’re different products.

1. I sell a product which I will describe in general terms. Anyone can buy it, but it’s pretty niche and unknown. There are a few other listings with this same product but mine is doing well because of pro pictures and the best pricing. That said, I don’t want someone else jumping in and selling the same thing.

It’s my understanding that to gate the product so only I can sell it, I would have to have a trademark and prove that this item is trademarked. Problem is, for our purposes here, it is essentially unbranded….and certainly I don’t have rights to it. I.e., I’m not Nike selling Nike shoes, I’m random seller selling basically generic but profitable niche product.

I created the catalog listing. There is no upc. So is there anything I could do to lock this listing down? Only thing I could think of would be to change the brand to my LLC name but then Amazon asked me to prove a relationship, which I couldn’t do. So i had to reverse the brand name change. I do not produce this item, I buy from supplier and send to FBA. But there is zero chance of the OEM coming after me. Hate to be vague but I’m protecting my research.

1. Somewhat related question. There’s an item I found in a local dollar/five below type store that is unbranded but does have “China” stamped on it. Can I sell this as my product? It has a (removable) sticker with a upc which IS in the Amazon catalog but my idea would be to market this in a different category on Amazon and possibly bundle it with another item which would somewhat change the functionality. Kind of like if I bought a Tupperware bottom and stretched a balloon over the top of it and called it a TupperDrum. If the Tupperware bottom and balloon have no branding, is this my protectable product?

As some of you know, it’s hard work finding a valuable product that meets the right parameters to invest in and I’d like to protect that by branding it but I’m not clear on if/how to do that if I haven’t manufactured the item myself. And it’s not clear what constitutes “manufacturing.”