Sometimes, they get them in bulk because they are replicas, and they have access to the distributor of these items.
In most states, as long as you have a resale license, you can buy anything associated with your business through a wholesaler (I used to work for a jewelery distributor, and we sold bulk items to different retail stores.)
Other people just spot good deals and take advantage of this. I have a friend who purchased a ton of authentic designer handbags from Lord & Taylor when they sold out in St. Louis. She was able to buy them on sale because she bought so many, but then was able to sell them for full price on Ebay. And this was 5 years ago, so as you can see buying from the pallet can really be worth it.