How the small town of Roundup, Montana, became a prominent place for 'prep centers,' and an important part of Amazon's supply chain

The Amazon ecosystem depends on "prep centers;" here's how Montana became a...

More than half of all product sales on Amazon come from third-party sellers who use the Amazon Marketplace to hawk their wares.

A lot of these sellers choose to participate in the Fulfillment by Amazon program so they can get Amazon's coveted Prime label and free shipping on their merchandise. Under this program, sellers ship their products to an Amazon warehouse and then the company fulfills the orders as Amazon customers place them. 

But before Amazon can accept these products into its warehouses, they have to meet dozens of guidelines.

That's where prep centers come in. 

Last month, CNBC flew to Roundup, Montana, to get a closer look at the role that these prep centers play in Amazon's vast supply chain. Watch the video to find out more.