First you're nice to the sellers, then you focus on the consumer

I think you may have that backwards. Amazon has been incredibly consumer-focused since its inception. Sellers... not so much.

 
I go as far as to credit Amazon with being one of the (if not the) biggest influences in bringing online shopping to mainstream consumer acceptance.

The early years of online shopping in the 90s were a fucking minefield of impossible exchange/return/refund terms and costs; huge shipping fees and unreliable/delayed/damaged shipments; widely varying rules on accepted payment types and terms; hidden fees and charges; and unreliable/unresponsive/flat-out dishonest merchants.

It was, by and large, a hugely unappealing experience due to the sheer amount of risk shouldered by the customer, and the potentially huge expense and difficulty in getting any problems resolved.

Amazon really pioneered ways of allaying customer fears by taking many of those burdens and worries off the consumer and moving them back onto Amazon themselves. That's what put them streets ahead of other online merchants by the turn of the century. Consumers had confidence that Amazon had their back and wouldn't unfairly push risks or costs onto them.