Amazon's emphasis on customer satisfaction is what mainly keeps me placing orders there. I know that if I have a problem with anything I buy there, I don't really have a problem, because Amazon will resolve the situation to keep me as a happy customer.
I'll occasionally shop around on the internet, and place orders for some stuff from other retailers, but if Amazon is close in price, Amazon is where I'll place my order. I've never ordered anything from the other retailers you mentioned except for two purchases of used snowblower parts from eBay. If you're right that they're attracting sellers that don't like the Amazon customer-satisfaction approach, I'm probably lucky I've never had to experience dealing with them.
Probably good that your one friend won't sell on Amazon anymore. Customers don't like the experience of having to fight to get their money back if they're not happy with what they received. From what you say, Amazon is just choosing to make it plain to their retailing partners... "if you want to fight with your customers, sell your wares elsewhere. If you understand the overall value of satisfied customers that will buy and buy, year after year, then you can sell your products through our site".
Amazon stole $200 from me when their seller went bankrupt.
No money. No product. An FU by Amazon.
I buy elsewhere. It will take 10 years to recoup my lost money.