I think that the seller issuing a Tax invoice to the buyer is really a flawed concept even if the seller is registered for GST.

The way I see it is that the buyer's transaction is really with eBay and not with the seller.

eBay bills the buyer for the item they are purchasing and the buyer pays money to eBay. The buyer doesn't pay the seller therefore the seller can't be invoicing the buyer.

This Managed payments system by eBay has really muddied the whole seller/buyer relationship. Before it, the buyer made the purchase and paid the item price to the seller's PayPal or another account. The seller then paid eBay for the services he/she used by the way of fees.

I don't mind the Managed payments now that I got used to it, but the occasional request from the buyer 'Please send a Tax invoice' drives my blood pressure up.