diff --git a/apps/api/src/modules/sales/domain/sale.js b/apps/api/src/modules/sales/domain/sale.js
new file mode 100644
index 0000000..766dee6
--- /dev/null
+++ b/apps/api/src/modules/sales/domain/sale.js
@@ -0,0 +1,99 @@
+function requiredText(value, fieldName) {
+  if (typeof value !== 'string' || value.trim().length === 0) {
+    throw new Error(`${fieldName} is required.`);
+  }
+
+  return value.trim();
+}
+
+function requirePositiveMinorUnit(value, fieldName) {
+  if (!Number.isInteger(value) || value <= 0) {
+    throw new Error(`${fieldName} must be a positive integer minor unit.`);
+  }
+
+  return value;
+}
+
+function requireNonNegativeMinorUnit(value, fieldName) {
+  if (!Number.isInteger(value) || value < 0) {
+    throw new Error(`${fieldName} must be a non-negative integer minor unit.`);
+  }
+
+  return value;
+}
+
+export function createSale({
+  sellerId,
+  saleDate,
+  saleLocation,
+  soldPriceMinor,
+  currency = 'USD',
+  platformFeesMinor = 0,
+  paymentProcessingFeesMinor = 0,
+  shippingCostMinor = 0,
+  taxesMinor = 0,
+  otherFeesMinor = 0,
+  buyerName,
+  buyerContact,
+  notes,
+}) {
+  return {
+    sellerId: requiredText(sellerId, 'Seller ID'),
+    saleDate: requiredText(saleDate, 'Sale date'),
+    ...(typeof saleLocation === 'string' && saleLocation.trim().length > 0
+      ? { saleLocation: saleLocation.trim() }
+      : {}),
+    soldPriceMinor: requirePositiveMinorUnit(soldPriceMinor, 'Sold price'),
+    currency: requiredText(currency, 'Currency'),
+    platformFeesMinor: requireNonNegativeMinorUnit(platformFeesMinor, 'Platform fees'),
+    paymentProcessingFeesMinor: requireNonNegativeMinorUnit(
+      paymentProcessingFeesMinor,
+      'Payment processing fees',
+    ),
+    shippingCostMinor: requireNonNegativeMinorUnit(shippingCostMinor, 'Shipping cost'),
+    taxesMinor: requireNonNegativeMinorUnit(taxesMinor, 'Taxes'),
+    otherFeesMinor: requireNonNegativeMinorUnit(otherFeesMinor, 'Other fees'),
+    ...(typeof buyerName === 'string' && buyerName.trim().length > 0
+      ? { buyerName: buyerName.trim() }
+      : {}),
+    ...(typeof buyerContact === 'string' && buyerContact.trim().length > 0
+      ? { buyerContact: buyerContact.trim() }
+      : {}),
+    ...(typeof notes === 'string' && notes.trim().length > 0 ? { notes: notes.trim() } : {}),
+  };
+}
+
+export function createSaleItem({
+  saleId,
+  inventoryItemId,
+  soldPriceMinor,
+  costBasisMinor,
+}) {
+  return {
+    saleId: requiredText(saleId, 'Sale ID'),
+    inventoryItemId: requiredText(inventoryItemId, 'Inventory item ID'),
+    soldPriceMinor: requirePositiveMinorUnit(soldPriceMinor, 'Sold price'),
+    costBasisMinor: requireNonNegativeMinorUnit(costBasisMinor, 'Cost basis'),
+  };
+}
+
+export function calculateNetSaleProceeds({
+  soldPriceMinor,
+  platformFeesMinor = 0,
+  paymentProcessingFeesMinor = 0,
+  shippingCostMinor = 0,
+  taxesMinor = 0,
+  otherFeesMinor = 0,
+}) {
+  const salePrice = requirePositiveMinorUnit(soldPriceMinor, 'Sold price');
+  const platformFees = requireNonNegativeMinorUnit(platformFeesMinor, 'Platform fees');
+  const processingFees = requireNonNegativeMinorUnit(
+    paymentProcessingFeesMinor,
+    'Payment processing fees',
+  );
+  const shipping = requireNonNegativeMinorUnit(shippingCostMinor, 'Shipping cost');
+  const taxes = requireNonNegativeMinorUnit(taxesMinor, 'Taxes');
+  const otherFees = requireNonNegativeMinorUnit(otherFeesMinor, 'Other fees');
+
+  return salePrice - platformFees - processingFees - shipping - taxes - otherFees;
+}
diff --git a/apps/api/tests/domain/sale.test.js b/apps/api/tests/domain/sale.test.js
new file mode 100644
index 0000000..75d492d
--- /dev/null
+++ b/apps/api/tests/domain/sale.test.js
@@ -0,0 +1,57 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+
+import {
+  calculateNetSaleProceeds,
+  createSale,
+  createSaleItem,
+} from '../../src/modules/sales/domain/sale.js';
+
+test('creates a valid sale record and calculates net proceeds after fees', () => {
+  const sale = createSale({
+    sellerId: 'seller-id',
+    saleDate: '2026-09-08',
+    saleLocation: 'eBay',
+    soldPriceMinor: 4999,
+    currency: 'USD',
+    platformFeesMinor: 300,
+    paymentProcessingFeesMinor: 120,
+    shippingCostMinor: 50,
+    taxesMinor: 90,
+    otherFeesMinor: 40,
+    buyerName: 'Alex',
+    buyerContact: 'alex@example.com',
+  });
+
+  assert.equal(sale.soldPriceMinor, 4999);
+  assert.equal(sale.currency, 'USD');
+  assert.equal(calculateNetSaleProceeds({
+    soldPriceMinor: 4999,
+    platformFeesMinor: 300,
+    paymentProcessingFeesMinor: 120,
+    shippingCostMinor: 50,
+    taxesMinor: 90,
+    otherFeesMinor: 40,
+  }), 4399);
+});
+
+test('rejects invalid sales and negative fee values', () => {
+  assert.throws(
+    () => createSale({
+      sellerId: 'seller-id',
+      saleDate: '2026-09-08',
+      soldPriceMinor: 0,
+    }),
+    /must be a positive integer minor unit/,
+  );
+
+  assert.throws(
+    () => createSaleItem({
+      saleId: 'sale-id',
+      inventoryItemId: 'inventory-item-id',
+      soldPriceMinor: 2000,
+      costBasisMinor: -1,
+    }),
+    /must be a non-negative integer minor unit/,
+  );
+});