Seller guides
Understand the numbers behind your sales.
Practical explanations to help you choose inputs, check costs and compare selling scenarios.
Reviewed 14 October 2026 · Free to useProfit margin vs ROI: which number should sellers use?
Understand the difference between profit margin and return on product cost, with a simple UK seller example.
VAT and marketplace fees: avoid counting tax twice
A practical explanation of VAT-inclusive selling prices, fee VAT and effective costs in marketplace profit estimates.
Amazon FBA size and weight: measure the packaged product
Learn how packaged dimensions, unit weight and dimensional weight affect the Amazon UK FBA estimate.
eBay postage and fees: revenue is different from delivery cost
Understand why postage charged to the buyer and the cost of sending a parcel need separate inputs.
TikTok Shop discounts and commission: follow the funding
See how seller-funded and platform-funded discounts affect a completed TikTok Shop UK sale.