It may depend on if the actual seller has a presence in NY. On my 2 most recent orders, one was taxed and the other wasn't but they were sold/shipped by 2 different vendors. I know one of the vendors DOES have a presence in NY (where I live) and that entire order was taxed.
It depends from whom you're buying the item. If it's an item that's sold by Amazon, you'll pay sales tax. If the item is sold by another vendor but fulfilled by Amazon you may not pay tax if that vendor is not based in, or have a business location in, New York.
Oops, just noticed the post above mine saying the same thing.