Distribution centers, outlets, etc. Once an item is sold through the catalog or store and then returned, most companies won't put it back out at full price, so they send it on to their distribution center where it's sold at a discount.
Other HUGE eBay sellers may have deals where they purchase directly from the corporation, but those are few and far between because they sell by the truckload literally.
I used to sell a huge volume of brand new clothing on eBay, in case you're wondering how I know all this. Nowadays I focus only on bags though