Vertical marketplaces are expanding again — Sigma Sports opens its platform to third‑party sellers Sigma Sports (UK) is launching a marketplace and is already recruiting specialist sellers ahead of its Summer 2026 rollout. Why it matters: broad marketplaces are not the only game. Vertical platforms can offer higher intent traffic and better conversion — but they enforce stricter standards (content, delivery, product authenticity). Playbook for sellers: start with 10–20 best-fit SKUs, build category-perfect listings (specs + sizing + photos), and set a clear returns policy.