-You can't block scammers -Buyers can return an item for any reason (even a year after they purchased it from you). 99% of the time, Amazon will let them keep the item and refund them. This happened to me many times. -If you get banned, they will keep your money for 90 days. Banning is based on automated robots that look for patterns. You have no recourse. All Amazon associates will tell you to use their email support system. The email support system is manned by either scripts or reps that will only respond with canned answers.
Ebay isn't nearly as bad. I've had claims filed against me a few times by various people and as long as I had the tracking number, I won the claim.
As someone who made amazon thousands of dollars, I got tried of being treated this way. The only reason they can still continue this behavior is because they are the biggest marketplace.
It's hard for competitors because you need the traffic.