Bootlegs on Mandarake do exist, not just in the Urban Mines auctions. Here's an old blog about it: (mfc link)
They are a second-hand store dealing with any kind of nerdy merchandise, all from old toys to newer figures. No second-hand store is perfect, a bootleg will eventually pop up every now and then. They are buying directly from private persons after all and no employee can keep track of every item and how to look for a bootleg for them.