Instead, I'll either avoid the independent sellers, or just to to Sur le Table. So how do these sellers win?
It's more of a platform to see what works and what doesn't, at the expense of small businesses who wanted to sell on Amazon. At least that's my perspective. A 3-5% margin may be fine when you run a 100B business, have economies of scale and a world-class brand, but the same will spell disaster for a small business.
There are also a lot of dishonest 3rd party sellers who come and go like a swarm of locusts. Maybe they "win" in the short run by selling counterfeit goods and not honoring Amazon policies, but in the end they will lose also when Amazon bans non-prime fulfillment of orders (that day is coming sooner rather than later, my guess is before 2020)
The majority of 3rd party sellers by volume own their own websites outside of Amazon. The ones you described are tiny mom&pop stores that come and go by the thousand every year.
In your view, how much should you have paid for shipping for an item that you didn't want? Nothing?
Should the seller be responsible for shipping the item both ways?
There is already very little margin in selling commodity goods, especially when you consider Amazon takes 15% of gross value as a fee. Shipping in many cases changes the order from profitable to money-losing. If you try and price it in, you can't compete on cost which begs the question of why bother selling on Amazon in the first place.
The crux of the issue is this, though: Amazon doesn't give a shit about 3rd party sellers, nor can the 3rd party sellers provide the same level of customer service as Amazon can, mostly due to economies of scale. The whole 3rd party seller program is a testbed for Amazon, a research program to see how they operate and how Amazon can do it better and as soon as they realize how - the 3rd party sellers either get priced out by a race to the bottom with Amazon itself (they go directly to suppliers and cut everyone out) or by policies like these, which are ripe for abuse or eat further into seller margins to the point of operating at break-even or slightly in the red.
As a last note: Whenever I buy on Amazon, I buy Prime. The incentives of customers and 3rd party sellers are incompatible. If I were a customer for the goods that we are selling, I would never buy from our company.