Where do these people on ebay get stuff at bulk and sell them for DIRT cheap?

I try to sell cell phones and can't compete with the prices. They sell NIB for cheaper than I am selling used. And someone has tons of mp3 players and tons of flash drives. Selling them free shipping and next to nothing at all. Where do they get it all from? I wanna sell on ebay too, but don't know where to get products from