I wouldn't consider it trolling if you can provide good evidence of the issues and not just opinion or hearsay. IMHO
It would be a good idea to PM the person selling the item to inform them first. They might have no idea of an issue if they have had no problems at all with the item they are selling. I know I would never want to knowing sell a product to anyone (let alone a fellow member of the Horde) without disclosing any known issues.