Validate the niche
Make sure there is a market.
Act as an e-commerce advisor. I want to sell [What you sell] to [Your target customer]. Assess the opportunity: who exactly buys this, what problem or desire it serves, 3 competitor types and how to differentiate, and 3 quick ways to validate demand before I invest heavily.
Pro tip: Differentiation matters more than a "perfect" product — know why a customer picks you over Amazon.