Yes, it is anonymous. When you report an item, only eBay gets to know who reported it, & they don't let on to the seller who reported him/her. This is whether you're eventually proved to be in the right or wrong.
But be aware that the final decision about an item's genuineness (or lack of!) lies entirely with eBay.
See Reporting inappropriate trading click here, & File a Report click here (this can also be accessed via the item's page). G