A possible explanation occurs to me. The King George VI tartan (STA 5845) is very similar to the Princess Mary tartan (STA 5449), which is associated with the Royal Family but is in regular unrestricted production. Both have been known as Green Stewart and differ only in the size of the white stripe.

It may be possible that the tartan was misrepresented by the seller on ebay through ignorance. However, against this is the point that the Princess Mary is associated more with women's items. Without a clear close sighting of the tartan we are taking the word of the seller.