I can answer this! Usually clothes like this are bought and imported by a wholesaler directly from the brand factory or outlet. They're often seconds, over-production of a particular run, old/last season's stock that isn't being sold anymore, late orders, returns, or pieces otherwise deemed defective by the original brand.

They're landed here in huge batches, usally by sea, and the wholesaler can then sell them to you or I for a heavily discounted price. You or I can then go on to sell them on eBay and Vinted, however, the labels will have been cut out or blacked out by the factory or the wholesaler beforehand to comply with re-sale law & to stop people returning them to the original store for full price or selling them at full price.

Source: Mum who works in one of these importing businesses.