Sellers often pick a platform by vibes — “Amazon has the buyers” or “eBay is cheaper.” The take-home number rarely matches either slogan.
Start with the same product cost, postage, and return rate on both channels. Use the marketplace fee calculator with VAT on platform fees enabled if you are a typical UK business seller.
Amazon’s referral fee looks simple until FBA fulfilment and returns show up. eBay business final value fees plus order fees can still win on thin-margin catalogue goods — or lose once you promote heavily.
Rule of thumb: never commit inventory until the expected profit after returns is still positive on your slower channel.