If nothing else, they at least allow you to filter out non-amazon (or even non-prime fulfilled) items.
- Search - Pick a category - Select "Amazon.com" as the seller
edit: That wasn't clear. Try two: Inventory submitted by FBA vendors is commingled with Amazon inventory in Amazon's warehouses. When you, a sucker, buys it, you have zero control over whether the widget you get was purchased by Amazon from the manufacturer or sent to Amazon by Billy Bob's Sellin' Totally Not Fake Shit Shack, which definitely got it from the manufacturer, not the bottom price on Alibaba. This is true even if you buy from Amazon and not an FBA vendor.
You can fairly consistently filter by Prime/non-Prime, but that's not the same (that filters down to things fulfilled by Amazon, but not just sold by Amazon).