Products not appearing to be sold on Amazon when scanned in the seller app?

I’m a new individual seller looking for products to list to get my foot in the door and i’m finding a lot of great deals on brands using Brick Seek but almost every item i scan on the app says it’s not listed on Amazon. What does this actually mean? Is it not allowed to be sold on Amazon at all? Is it restricted? Should i stay away from products that aren’t already on Amazon?