Amazon Marketplace is an e-commerce platform owned and operated by Amazon.com. It enables third-party sellers to sell new and used offerings on Amazon.com's fixed-price online marketplace, alongside Amazon's regular offerings.
Using Amazon Marketplace, third-party sellers gain access to Amazon's customer base, and Amazon expands the offerings on its site without having to invest in additional inventory. Customers gain access to additional offerings, increased competition, and the ability to purchase from third-party sellers.
The GS1 system: For most product categories, sellers are required to use standardised GS1 system product identifiers. Read more
eBay Inc. is a global commerce leader including the Marketplace, StubHub and Classifieds platforms. Collectively, we connect millions of buyers and sellers around the world, empowering people and creating opportunity through Connected Commerce. Founded in 1995 in San Jose, Calif., eBay is one of the world's largest and most vibrant marketplaces for discovering great value and unique selection. In 2015, eBay enabled $82 billion of gross merchandise volume.
The GS1 system: ebay states that GTINs are one of the required product identifiers mandatory for listing within specified categories. Read more
Google Shopping is “a Google service … which allows users to search for products on online shopping websites and compare prices between different vendors.” 
The GS1 system: Valid GTINs are required for all Google Shopping feeds targeting Australia, Brazil, Czech Republic, France, Germany, Italy, Japan, Netherlands, Spain, Switzerland, UK and the US. Read more