Sorry I don't buy that. The only reason a consumer would choose a 3rd party seller is price. If Seller can beat Amazon (presumably the most efficient retailer on Earth) on price then Seller must have obtained the goods at below wholesale price.
What value do 3rd party sellers actually provide to Amazon customers? Yes sometimes the price is lower, but that's usually because they bought the goods at a retail store auction for next to nothing (these auctions sell lots of returned merchandise sight unseen). Anyone running one of these businesses has to realize what they're doing is more than a little sketchy.
Frankly I don't see the benefit in this program for Amazon. 3rd party sellers often charge less, meaning less revenue for Amazon. The highly variable customer service also degrades Amazon's brand. Finally, Amazon still has to front the cost of any return/refund from a 3rd party seller, so there is still risk.
From the PoV of a company whose products get sold by 3rd party sellers on Amazon, it's a total disaster. They regularly sell used or returned items as 'new,' and most customers don't notice. They provide atrocious customer service. Amazon does nothing to police this. These sellers also do everything in their power to avoid processing a return. I don't want to crap on your business, but I hope these regulations push the less scrupulous 3rd party sellers back to eBay.
As a hardware startup who sells on Amazon, 3rd party sellers are a total nightmare. They hijack your Amazon product page, and often sell returned merchandise as 'new other' at a significant discount. Customers typically don't notice the distinction, and think they are just getting a sweet deal on a new product. I'm glad Amazon is imposing more restrictions on 3rd party sellers. Hopefully this will force some of the junk sellers to move to eBay.