It's the opposite of what you're saying. Sellers not selling particular items on Amazon is good for consumers who are price conscious, because if they need that particular item they will search for it and find it cheaper on another store, strengthening that one and hurting Amazon.
And if they don't care they will find cheaper items on Amazon. Everyone wins except for the seller, which is Amazon's MO.
Sellers don't just 'not sell this item on Amazon', because it is the largest single market. This is like, the whole issue in question.
> because if they need that particular item they will search for it and find it cheaper on another store, strengthening that one and hurting Amazon.
This is the exact thing that isn't happening, because Amazon is abusing its market position.
But, even if this was the case and the seller chose to sell on a different marketplace and take the Amazon ban, then it still wouldn't be as good as having just having the item listed on both marketplaces at two different prices. Because then I wouldn't be able to buy the best lightbulbs in one convenient location.
So here's a weird externality. As a decently well off tech-worker, I don't care if I'm paying 5$ or 2$ for lightbulbs, even if I'm re-lighting my whole house. The seller will sell on Amazon, because it is the largest market and will set their prices on other markets to match Amazon. I'll buy on Amazon, because it isn't worth my time to check multiple websites to get the best price (especially when SKUs differ and I have multiple other items I'm buying, etc). And now people who do need to save money, can't also find this lightbulb at a cheaper price on competing marketplaces, even if this lightbulb is actually the best quality.