Costing one England marketplace seller workflow, from price cards to exit

This marketplace seller software cost guide builds one whole-life ledger for a fictional England merchant. The job moves an approved product record and stock position from the seller's source system to one authorised marketplace account, then returns later order events.

No price, wage, volume or saving was available at the research cut-off of 6 September 2026. Use blank fields until an accountant and procurement owner approve the evidence.

Define the units first

Name the legal seller, product population, account, source system, review period and currency. Split later events into placed, accepted, payment authorised, captured, fulfilled, retained, cancelled, returned, partly or fully refunded, disputed and settled. A single "per order" label is unusable unless the contract identifies which event triggers the charge and what happens after reversal.

For each supplier amount, capture exact legal entity, service and edition, UK eligibility, currency, VAT status, unit, billing period, quantity, minimum term, limits, refund or dispute treatment, promotion, exclusions, source and access date. If one field is absent, mark the amount HOLD.

Build cost rows around work

Use separate rows for discovery, product-data repair, configuration, migration, interface work, testing and training. Add recurring software, hosting, extension, support and managed-labour rows. Then record internal reconciliation, access administration, failed-event handling, customer support, correction and monitoring.

The ledger also needs consumer, safety, privacy, security, accessibility, contract, accounting and tax assurance. GOV.UK's online-selling guidance identifies price, product, correction and confirmation matters that may require evidence and review. These tasks do not disappear because a supplier includes a checkout field.

For goods, include traceability, warning, incident and withdrawal work. OPSS product-safety advice preserves the Great Britain and Northern Ireland distinction and notes the possible priority of product-specific rules. Keep this gate outside any cost score.

Separate cash and economic cost

Cash rows show payments and receipts in their actual period. Accounting rows follow the accountant's treatment. Economic rows add internal time and displaced work from approved organisational evidence. Do not invent an hourly rate or treat salaried time as free.

Use this blank formula:

Whole-life cost = setup + recurring service + internal operation + assurance + exception handling + change + exit

Each term needs currency, VAT basis, unit, quantity, period, source, owner, confidence and scenario. The HM Treasury Green Book 2026 is public-sector guidance, not a rule for this merchant. Its treatment of business as usual, lifetime costs, risks and uncertainty is a useful method reference only.

Record VAT as a reviewed field

HMRC's VAT-record guidance covers supplies, adjustments, invoices and credit or debit notes. Supplier VAT, marketplace transaction VAT and cash settlement are not the same line. Enter gross amount, net amount and VAT effect only after the adviser confirms status and source.

Include exit before approval

Budget for export, open-order handover, correction access, isolated restore, account removal, retention, deletion and return to the current process. NCSC supplier-assurance questions include continuity and contract-exit prompts; they do not establish a cost or assurance result.

The budget decision is pass, rework, hold or stop. Hold while a monetary field lacks evidence, and stop if any consumer, safety, tax, privacy, security, accessibility or exit gate fails. This guide is not a quotation or tax calculation.