As above, a few too many brand new items in higher risk categories (mostly with stock photos) likely got you flagged.

The request for invoices were in part to see what the nature of your selling likely was ie private seller or business and whether you had the items in hand or not. It was a fait accompli I think it will be difficult to successfully appeal. If you are going to pursue it, I’d suggest ringing and speaking to one of their agents by phone.