Why not practically everything?
Why should we be the only sellers of these items? Millions of third-party merchants and small sellers offer similar or unique items. Why not let customers choose the selection, price, and delivery speed they prefer from among these millions of sellers?
After struggling for a couple years to create awareness for sellers' selection, we asked ourselves why not show their selection on the same product detail pages as our first-party selection (where all the traffic was)?
Why not allow our sellers to also store items in our fulfillment network, enable those items to have fast, Prime delivery, and fulfill those items for sellers (a program called Fulfillment by Amazon)?