You tend to do deals with people you have an affinity with, especially at the beginning.
Something like Pinterest would scare the hell out of me. With $5 billion valuation - regardless how sound those numbers are - wouldn't be one that I would start, given what I would consider the risk of failure.
You have to have a unique product that Amazon just can't source. It's all about the product. Why is it important? Why is it different? Amazon are not merchants. They are technology platform guys.
Amazon has reported a loss of nearly $900M in 1999... because, guess what, ecommerce is a business of scale. And, the Internet bubble had burst... which meant that no one was getting funded.
Here is what the world looked like in 2000... there were no plug and play solutions for ecommerce/warehouse management and customer service that could scale... which means that we had to employ 40+ engineers. Cloud computing did not exist, which means that we had to have a server farm and several IT people to insure that the site did not go down.
Why are people so obsessed with Pets.com? We shut it down and returned money to shareholders. Besides, there were plenty of other dot-com failures around then.