Independent (non-aggregator) web sites generally have some control over what suppliers they draw from. So of course there are counterfeits out there, but they’re the exception, unless selling counterfeits was the seller’s intention. You can generally judge based on how reputable the seller is.

Amazon sources indiscriminately, because they’re not a seller in the traditional sense; they’re basically a flea market with warehousing and logistics. So you have no guarantees one way or another whether the thing they’re selling is genuine or not.