just to clarify, it is only sold directly from Amazon themselves if it says "ships from and sold by Amazon.com". any seller who is not "amazon" themselves is a third party seller, even the ones fulfilled by amazon. it is easy to tell who is who when you click on the options on the page for a particular item. usually there will be an link that says "used and new from $##" which will allow you to pick a different seller if one is available. on that list of different sellers offering the item at different prices (and in different conditions), there is always a seller username on each one. each seller has a unique id like on eBay made up of numbers and/or letters. amazon is the only seller without an actual "username" in the classic sense. when amazon is selling, the username slot is just occupied by the amazon logo. unless you see the amazon logo where the seller name is on the page with the list of options, it is NOT sold by amazon.com. and "fulfilled by amazon" only means that amazon is willing to either do prime shipping on it, and/or will assist you to the extent of their (amazon.com's) own policies if something goes wrong with the sale. however, the person doing the actually selling is a third party
the main real concern here should be if the "co-mingling", as described by other users, is true. that would screw everything I just said above. I assume though that amazon would feel they have to uphold a reputation and the risk of getting counterfeits from them should be low as a result