Do I have to report marketplace sales on my HST return even if the platform already remitted the tax?

It depends on whether you're the one registered and responsible for those specific sales, or whether the platform is the party the rules make responsible for them instead. If you're a GST/HST-registered Canadian seller and the sales are your own, not sales the platform rules have shifted onto the platform as the deemed supplier, those sales generally still belong on your own HST return, reported and remitted by you, regardless of how the platform processes payment or displays tax at checkout.

Where the platform is genuinely the party responsible under the rules, typically for certain non-resident vendor or short-term accommodation situations, the platform's own remittance covers that specific tax obligation, and you generally wouldn't separately remit the same tax again, but you may still have your own reporting or record-keeping obligations related to those transactions, depending on your role in the arrangement.

Because mixing up which category your sales fall into can lead to either double-remitting tax that was already handled by the platform, or failing to report sales you're actually responsible for, review your specific marketplace arrangement and seller status with a tax advisor so your HST return accurately reflects what you owe versus what the platform has already covered.