diff --git a/frontend/src/components/modals/ConfirmBuyModal.jsx b/frontend/src/components/modals/ConfirmBuyModal.jsx
index a9fa1e6..0867759 100644
--- a/frontend/src/components/modals/ConfirmBuyModal.jsx
+++ b/frontend/src/components/modals/ConfirmBuyModal.jsx
@@ -31,17 +31,17 @@ export default function ConfirmBuyModal({
   const hasPrev = currentIndex > 0;
   const hasNext = currentIndex < allItems.length - 1;
 
-  const handleIncrement = () => {
-    if (!isSubmitting && quantity < maxQuantity) {
-      setQuantity((prev) => prev + 1);
-    }
-  };
+  const handleIncrement = useCallback(() => {
+    if (isSubmitting) return;
 
-  const handleDecrement = () => {
-    if (!isSubmitting && quantity > 1) {
-      setQuantity((prev) => prev - 1);
-    }
-  };
+    setQuantity((prev) => Math.min(maxQuantity, prev + 1));
+  }, [isSubmitting, maxQuantity]);
+
+  const handleDecrement = useCallback(() => {
+    if (isSubmitting) return;
+
+    setQuantity((prev) => Math.max(1, prev - 1));
+  }, [isSubmitting]);
 
   const handleConfirm = async () => {
     if (isSubmitting) return;
@@ -88,12 +88,24 @@ export default function ConfirmBuyModal({
       if (event.key === "ArrowRight") {
         event.preventDefault();
         handleNext();
+        return;
+      }
+
+      if (event.key === "ArrowUp") {
+        event.preventDefault();
+        handleIncrement();
+        return;
+      }
+
+      if (event.key === "ArrowDown") {
+        event.preventDefault();
+        handleDecrement();
       }
     };
 
     window.addEventListener("keydown", handleKeyDown);
     return () => window.removeEventListener("keydown", handleKeyDown);
-  }, [handleNext, handlePrev, isSubmitting, onCancel]);
+  }, [handleDecrement, handleIncrement, handleNext, handlePrev, isSubmitting, onCancel]);
 
   const imageUrl = item.item_image && item.image_mime_type
     ? `data:${item.image_mime_type};base64,${item.item_image}`
diff --git a/frontend/tests/location-map-manager.spec.ts b/frontend/tests/location-map-manager.spec.ts
index 7fb8dde..f250cb6 100644
--- a/frontend/tests/location-map-manager.spec.ts
+++ b/frontend/tests/location-map-manager.spec.ts
@@ -3846,9 +3846,14 @@ test("viewer advances map buy modal after partial quantity buys", async ({ page
   await page.getByRole("button", { name: "Mark flour bought" }).click();
   await expect(page.locator(".confirm-buy-item-name")).toHaveText("flour");
 
-  await page.getByRole("button", { name: "Decrease quantity" }).click();
-  await page.getByRole("button", { name: "Decrease quantity" }).click();
-  await expect(page.getByRole("spinbutton", { name: "Quantity to mark bought" })).toHaveValue("1");
+  const quantityInput = page.getByRole("spinbutton", { name: "Quantity to mark bought" });
+  await page.keyboard.press("ArrowDown");
+  await page.keyboard.press("ArrowDown");
+  await expect(quantityInput).toHaveValue("1");
+  await page.keyboard.press("ArrowUp");
+  await expect(quantityInput).toHaveValue("2");
+  await page.keyboard.press("ArrowDown");
+  await expect(quantityInput).toHaveValue("1");
   await page.getByRole("button", { name: "Mark as Bought" }).click();
 
   await expect.poll(() => boughtPayloads.length).toBe(1);