First, you have to differentiate between buying shoes from Amazon, and buying shoes at Amazon.com. Amazon sells products direct, and they permit lots of third parties to advertise on their website, and even do the fulfillment (billing and shipping) for some.
If you are buying from Amazon, there is almost no risk of fakes. They are very careful with their reputation and have the clout to cut off contracts with suppliers if any fakes slip in.
If you are buying on the amazon.com website, there is just a bit more risk, but very, very little, really. Amazon keeps their third parties on a short leash and on the straight-and-narrow (choose your metaphor).
If you are at all concerned, here is a very good explanation of how to spot fake Nikes, as a for instance:
https://www.wikihow.com/Spot-Fake-Nikes#:~:text=Examine%20the%20SKU%20number%20on,Check%20the%20tongue%20label
I have bought many, many pairs of running shoes both from Amazon and on Amazon, as well as from Zappo’s, RunningWearhouse, and others, and I have never been disappointed. And, for what it’s worth, I also bought a pair of Nikes from an outfit called Running Dervish, operating out of a post office box in (I think) Hoboken, New Jersey. Shoes were perfect, and arrived a day early.