Choose the unit of comparison
Use a stable SKU or product identifier, variation, currency, stock status, seller and tax context. Record shipping separately. A displayed headline price is not necessarily the price at checkout, and a failed response does not mean an item is unavailable.
Control location and session state
Residential can suit a confirmed regional view; ISP can suit a workflow that needs a consistent allocated set. Cookies, account state, delivery postcode and browser language can affect the result independently of the exit IP. Keep these inputs fixed in a permitted test.
Protect your budget
Collect only the needed pages and fields. Cache unchanged responses where allowed, honor rate limits and use bounded retries. Estimate response bytes including images or rendering before requesting a commercial configuration; proxy usage and item count are different units.
Report useful differences
Store source, timestamp and normalized currency alongside the original displayed value. Flag tax or stock uncertainty instead of silently filling gaps. Avoid real orders, cart manipulation, account creation or other transactions on third-party services.