Build vs Buy · Build vs Buy · Medium · 25-35 min

E-commerce Platform — Build, Buy, or SaaS

E-commerce Platform — Build, Buy, or SaaS is a medium Build vs Buy build vs buy case interview that runs 25-35 min. A $500M-revenue retailer's e-commerce platform is creaking and limits growth. A strong answer works through 5 phases: Define requirements; Compare SaaS vs packaged vs custom on cost, speed, flexibility, and operational burden; Build the 5-year TCO including subscription/transaction fees vs build-and-maintain cost; Assess customization ceiling of SaaS vs the maintenance burden of custom; Recommend by fit.

Last updated 2026-09-05

The brief

A $500M-revenue retailer's e-commerce platform is creaking and limits growth. Options: adopt a hosted SaaS commerce platform, license a packaged platform to self-host, or build custom. The CMO wants speed and rich features; engineering wants control; finance wants the lowest 5-year cost. You advise.

How to approach it

  1. Define requirements — scale, customization needs, integration footprint, peak traffic
  2. Compare SaaS vs packaged vs custom on cost, speed, flexibility, and operational burden
  3. Build the 5-year TCO including subscription/transaction fees vs build-and-maintain cost
  4. Assess customization ceiling of SaaS vs the maintenance burden of custom
  5. Recommend by fit — most mid-market retailers don't need custom

What a strong answer does

  • Matches the answer to actual customization and scale needs, not preference
  • Recognizes SaaS transaction fees can dominate TCO at high GMV
  • Weighs the hidden cost of maintaining custom software over 5 years
  • Considers integration with ERP, payments, and fulfillment
  • Defaults to SaaS/packaged unless genuine differentiation requires custom

Red flags interviewers score down

  • Recommends custom build without justifying the differentiation it buys
  • Ignores SaaS transaction/percentage-of-GMV fees in the TCO
  • Underestimates the long-run maintenance cost of building
  • No view on integration with existing systems

Cases are written in each firm's style, written and reviewed by working consultants; they are not the firms' own published cases.

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