This should be Walmart's lane: no third-party sellers, just the best place to buy branded, verified products.
IKEA has good products but they don't come close to general merchandise stores like Amazon or Walmart.
Enable high quality sellers to have their own branded website to sell their own product directly.
Then make it easy to find the Shopify sites via some general search or directory.
You're able to slowly build up power this way without it being too obvious, and you provide an immediately valuable service to sellers that aligns incentives from the beginning.
[0] https://www.supplychaindive.com/news/walmart-ecommerce-fulfi...