The short answer
Compare marketplaces by contribution margin after fees, discounts, media, fulfilment, returns, and support. Validate one channel and a small SKU set through repeat purchase before expanding.
The best marketplace is not the one with the highest gross sales. Compare selling price minus product cost, platform fees, discounts, media, fulfilment, returns, and support to find contribution margin by channel.
Platforms differ in discovery mechanics and operating burden. A promotion can lift revenue while losing money after discounts and media, while strong conversion can be offset by return and review-management costs.
Calculate break-even ROAS, allowable acquisition cost, post-return margin, and time to repeat purchase by channel. Use SKU and campaign data rather than a blended average.
Start with one channel and a small SKU set. Expand only after demand, product-page conversion, operations, contribution, and repeat purchase conditions hold together.