The "fulfilled by amazon" part is pretty obvious on the page, so I wouldn't say it's hard to figure out. If they have it in one of their warehouses, they'll ship it to you and will refund you directly without getting the seller involved if they did and there is a problem with it - sometimes without even verifying you returned it (this has not happened to me but I've seen it happen to another person). If you chose to buy something that isn't in one of your warehouses, your mileage will vary.

Recently I got them to refund me after showing I returned the defective item (tracking number + signed by an employee) the SAME day it was returned to the seller's location. Granted, the seller and I had a dispute (which was that the seller refused to acknowledge the device spit out a fair amount of magic smoke when powering a small charger).

Again, it's effort in and effort out. If you buy something off Amazon, make sure it's being shipped to you by Amazon!

Does filtering by prime only help? I've started only buying prime eligible items and I've seen quality increase slightly. Also, oddly enough I've found jet.com to be far superior to Amazon in both price and quality when they actually have the item I need.