So after a lot of back and forth I have finally gotten to the bottom of the problem.
Customer wanted a straw hat. My hats are made of simulated straw (and says so in description). So their actual complaint was bogus as they had not read the description correctly and the fact they used 'inauthentic' in their complaint caused so much aggravation. The customer was just a normal customer - they had no authority on the issue, were not brand holder or anything so for Amazon to give any credence to such a complaint in the first place was a bit ridiculous.
Nevertheless, Amazon wanted last 365 days invoices from my supplier. I got my supplier to send me copies of all invoices from May 2023. I would send invoices, Amazon would come back to say they could not verify supplier (they had made no attempt at even contacting supplier as I asked the supplier) so then Amazon wanted suppliers supplier invoices. I would explain there was no suppliers supplier as I bought directly from the manufacturer who designed and produced the hat. Then Amazon would ask for last 365 days invoices, I provided and whole sequence kept repeating.
Finally an Account Health Specialist I spoke to (for the third time on the issue) said that if I went to Manage Orders, and searched last 365 days for the ASIN that the number Amazon said I had sold was greater than the number of units I had bought on the invoices.
So I checked - the trouble was
1. As Amazon kept asking for invoices from last 365 days and that is what I provided - items sold May 2023 were actually bought in April 2023 - so these invoices were missing and this stock meant my total bought was higher than Amazon calculated just from looking at last 365 days. So I provided invoices from April 2023 to May 2024 instead
2. Looking at the sales Amazon said I had made I realised they were counting orders made - not sales. There were many refunds where customer had returned the hat and it had then gone back into stock and re-sold. So the number reported by Amazon was higher than actual sales.
Taking into account units bought in April 2023 and the returned/re-sold items and it proved I had bought more units than had actually sold.
When appealing , Amazon only allows files to be uploaded and has no box for additional information. So I created a Word document containing the above information with actual figures showing my calculations and finally Amazon have agreed I can continue selling the item.
So for a month now I have been tied up on the issue. And all because Amazon gave credence to 1 complaint out of 1000 from someone who had no authority on the issue - and because Amazon could not understand basics of having to buy in stock before it can be sold (hence they needed more than last 365 days invoices) and the fact they were counting orders made and not actual sales.