This concept I believe represents a good enough sample size to provide a good overview of "most commonly linked-to items." If 100 people link to the same produc

This concept I believe represents a good enough sample size to provide a good overview of "most commonly linked-to items." If 100 people link to the same product across many domains, chances are that most of those will be Amazon links. You don't get all of the links, but you get most of them. The exception will be items from sellers of products not available on Amazon. And there may be some communities in Reddit that are more loyal to another e-commerce platform or site you would miss (crafters to Etsy, antique collectors to eBay, etc.)