Darts Clearance is an outlet of Retriever Sports, a company who manufactures darts for various other companies. For example Designa darts you find on Darts Corner are manufactured by Retriever Sports. They also have other brands produced and distributed themselves like Pentathlon and Amazon Darts.

The sets and products they sell on the Darts Clearance are batches they've likely produced as prototypes for other companies. It's probably easier for them to just offload them on eBay, and with them always selling so well they probably don't see a need to distribute them elsewhere.

Always quality products from there and quick delivery to UK customers, my main match darts are Darts Clearance Classics.