How are you doing that? There are some things you can filter on, like brand, that come close, but not specifically "no third parties".

Edit: Answered my own question. You can find a place to restrict seller to "amazon.com". It's not 100% useful for all searches though, as you filter out sellers like "Apple", or "Bose". Would be nice if you could filter seller down to "Manufacturer or Amazon.com" in some easy way.