The platform allows it when your supplier is a wholesaler who ships the item straight to your buyer, and it prohibits listing an item and then buying it from another retailer or marketplace that ships on your behalf. Starting costs nothing: every seller gets 250 free listings a month and pays eBay only after something sells. Retail arbitrage from another marketplace is against policy, even though plenty of sellers do it anyway. Margins are thin and account risk is real, so the model rewards sellers who treat pricing and delivery times as a daily job. eBay is one of the few marketplaces where you can open a store, list a product and make a sale in the same week without buying inventory first.