E-commerce and online sales

You are an e-commerce consultant. Help me with: [SITUATION — start selling online/increase sales of existing store/decide between marketplace, own store or social media]. My context: [PRODUCT, AVERAGE TICKET, BUDGET, CURRENT STRUCTURE]. Deliver: the right channel strategy for my product and moment (marketplace to start with ready traffic vs. own store for margin and brand — with the numbers of comparison: commissions, traffic, control), the essentials of operation that beginners underestimate (shipping, packaging, delivery time, returns, customer service — where bad reviews are born), the basics of traffic and conversion prioritized by my budget, real margin accounting per channel (including fees, shipping and tax), and realistic goals for the first 90 days. Objective: sell with margin, not just turn over revenue.