"Drop-shipping" via a retailer is not allowed, and you can report the seller to eBay for that.
https://www.ebay.com/help/action?topicId=4850
If the seller sells on both eBay and Amazon, and keeps stock in Amazon's warehouse that is used to fulfill orders, that is OK with eBay and Amazon.
If the seller has no stock, but waits until an order is placed to turn around and buy the item from Amazon and have it shipped to the buyer, eBay considers that "retail arbitrage" and it is not allowed.
If the seller is using a personal Amazon Prime account to place the order, the seller is likely violating the Prime agreement against using a personal account for commercial purposes.
https://www.ebay.com/help/selling/posting-items/setting-postage-options/drop-shipping?id=4176
