I ordered from Amazon's listing here, and made sure the listing had the item as "Sold by Amazon", which according to iRobot, is an authorized reseller, so Amazon should not be selling counterfeit / knockoff iRobot products when the product is listed as "Sold by Amazon".

The only ones you can trust are the ones that say "Sold by" and "Ships From" Amazon. Those are from iRobot. In those cases, Amazon acts as distributor for the brand. I've purchased bags and filters for my i4+ from the "Sold By and Ships From" on Amazon and they're both authentic.

Third-party seller items are not from iRobot authorized sellers.