diff --git a/code/in-search-of-a-unit/test/Auction.test.ts b/code/in-search-of-a-unit/test/Auction.test.ts
index 5539a54..0a81743 100644
--- a/code/in-search-of-a-unit/test/Auction.test.ts
+++ b/code/in-search-of-a-unit/test/Auction.test.ts
@@ -4,6 +4,7 @@ import { Money } from "../src/Money";
 import * as uuid from 'uuid';
 import { Offer } from "../src/Offer";
 import * as assert from "assert";
+import * as sinon from "sinon";
 
 function testOffer(amount: number, currency = 'PLN', user?: string): Offer {
     return {
@@ -26,25 +27,36 @@ function stuffAuction(auction: Auction, count: number) {
 describe('Auction', function() {
     it('should immediately reject an offer that could not possibly win', async function() {
         const notifier = new EmailNotifier();
+        const mockNotifier = sinon.mock(notifier);
         const sale = new Auction('PLN', 5, notifier);
         // Initially, everybody goes into auction at 10 PLN:
         const initialOffers = stuffAuction(sale, 5);
         // Subsequent offers for the same amount should be rejected:
-        const sixthOfferResult = await sale.makeOffer(testOffer(10));
-        assert.strictEqual(sixthOfferResult, false);
+        const sixthOffer = testOffer(10);
+        mockNotifier.expects('notifyUser').once().withArgs(sixthOffer.madeByUserID, {
+            eventType: 'outbid',
+            offer: sixthOffer
+        }).resolves();
+        const sixthOfferResult = await sale.makeOffer(sixthOffer);
+        mockNotifier.verify();
     });
     it('should replace the last offer if a better one comes in', async function() {
         const notifier = new EmailNotifier();
+        const mockNotifier = sinon.mock(notifier);
         const sale = new Auction('PLN', 5, notifier);
         const initialOffers = stuffAuction(sale, 5);
         // A new offer should annul the last one made:
         const lastOffer = sale.getQualifyingOffers().pop()!;
         const betterOffer = testOffer(15);
+        mockNotifier.expects('notifyUser').once().withArgs(lastOffer.madeByUserID, {
+            eventType: 'outbid',
+            offer: lastOffer
+        }).resolves();
         await sale.makeOffer(betterOffer);
-        assert.ok(sale.getQualifyingOffers().includes(betterOffer));
-        assert.ok(!sale.getQualifyingOffers().includes(lastOffer));
+        mockNotifier.verify();
     });
     it('should select the winners after some offers are replaced', async function() {
+        // TODO: Rewrite this test to use behavior verification as well!
         const notifier = new EmailNotifier();
         const sale = new Auction('PLN', 5, notifier);
         const initialOffers = stuffAuction(sale, 5);
diff --git a/code/package-lock.json b/code/package-lock.json
index 021401b..113b742 100644
--- a/code/package-lock.json
+++ b/code/package-lock.json
@@ -62,6 +62,21 @@
       "integrity": "sha512-Zw1vhUSQZYw+7u5dAwNbIA9TuTotpzY/OF7sJM9FqPOF3SPjKnxrjoTktXDZgUjybf4cWVBP7O8wvKdSaGHweg==",
       "dev": true
     },
+    "@types/sinon": {
+      "version": "9.0.8",
+      "resolved": "https://registry.npmjs.org/@types/sinon/-/sinon-9.0.8.tgz",
+      "integrity": "sha512-IVnI820FZFMGI+u1R+2VdRaD/82YIQTdqLYC9DLPszZuynAJDtCvCtCs3bmyL66s7FqRM3+LPX7DhHnVTaagDw==",
+      "dev": true,
+      "requires": {
+        "@types/sinonjs__fake-timers": "*"
+      }
+    },
+    "@types/sinonjs__fake-timers": {
+      "version": "6.0.2",
+      "resolved": "https://registry.npmjs.org/@types/sinonjs__fake-timers/-/sinonjs__fake-timers-6.0.2.tgz",
+      "integrity": "sha512-dIPoZ3g5gcx9zZEszaxLSVTvMReD3xxyyDnQUjA6IYDG9Ba2AV0otMPs+77sG9ojB4Qr2N2Vk5RnKeuA0X/0bg==",
+      "dev": true
+    },
     "@types/uuid": {
       "version": "8.3.0",
       "resolved": "https://registry.npmjs.org/@types/uuid/-/uuid-8.3.0.tgz",
diff --git a/code/package.json b/code/package.json
index cf2cade..106ae5a 100644
--- a/code/package.json
+++ b/code/package.json
@@ -19,6 +19,7 @@
     "@types/big.js": "^6.0.0",
     "@types/mocha": "^8.0.4",
     "@types/node": "^14.14.7",
+    "@types/sinon": "^9.0.8",
     "@types/uuid": "^8.3.0",
     "ts-node": "^9.0.0",
     "typescript": "^4.0.5"