Each listing can have a private cost price, regular selling price and optional sale price. Cost never appears publicly.
A promotion can run on Marketplace, POS, invoices, or any combination, with optional start and end dates. Outside the window the regular price returns. Sale price must be below regular price and at least one channel selected. Marketplace displays the regular price struck through and active sale price. Checkout, POS and invoices recalculate price server-side.