Per Amazon's policy - no, the seller can not have lower prices on their own website than on Amazon. In practice, for now, it is not enforced (at least in my experience).
most obvious alibaba or aliexpress goods on Amazon can be bought cheaper from one of the other places, and if it's a weird item (like a bicycle hub motor) it'll generally be from the same merchant group.
usually the copy/paste pictures and description are a good giveaway.
