I think trading standards would disagree. Knowingly handling / transporting counterfeit goods would be an offence in itself.
If you had your own shop and had any stock which was reported to you as ‘inauthentic’ they’d expect you to stop selling it immediately.
They wouldn’t expect you to just take it to your supplier and hand it back, they’d either want you to dispose of it (after reporting it to them for investigation), or just hand it over to them for testing.
Given the size of Amazon, the question is whether trading standards could ever handle the amount of enquiries if Amazon reported every case.