PlazMarket: a German marketplace where the law is part of the domain model

The hard part of a German marketplace is not the shopping cart. A German price label has to answer questions an ordinary e-commerce schema never asks.

What does this cost per kilogram — based on the price the shopper can actually get today, not the list price? How much of the total is a refundable container deposit, which is not part of the product's price at all? If it was on offer last week, what was the lowest price in the thirty days before? Get any of these wrong and the number on the page is not merely untidy — it is a misstatement of price.

The same is true after the sale. An invoice must carry a gapless number and mandatory fields, and if anything is refunded, a correction document must repeat those particulars. Money is split across sellers, held in reserve against returns, and paid out — and every step has to produce entries a tax audit can follow and that no code may quietly rewrite.

On top of all of it, the site has to work in five languages, two of which — Persian and Arabic — are written right to left.

Instead of bolting the law onto the system, it was written into the domain model. The unit price is computed from the price actually payable, so a campaign carries the per-kilo figure with it. The deposit is a domain value with its own VAT treatment, kept separate from the cart quote through the invoice line to the refund. Price history is recorded per offer so the thirty-day figure reads correctly. The return window comes from a policy tier snapshotted onto the order at the moment of sale, so a later policy change cannot alter what was promised.

Money is Money throughout rather than decimal, with one rounding rule applied once per line; and totals are carried forward from the quote into the order, the invoice and the settlement instead of being recomputed at each step.

Three products were built on one platform: a storefront with faceted search on Elasticsearch and multi-seller orders split into independent parcels; a seller panel with KYB onboarding, bulk CSV import, campaigns, fulfilment and returns, commission, reserve holds and payout statements; and an operator panel with catalog governance, an invoice approval queue, VAT by rate and payout execution.

GDPR and DSA compliance is structural in the same way: a subject-access pipeline fans out over every context with an email-verified identity check and an SLA clock; seller legal identity is public on the shop page; and an abuse report is routed to a moderator rather than triggering an automatic takedown.