1 · Observe
Record the rating and the review count for a watched item on a regular cadence, and record when each observation was made.
How it would work
A sketch, not a specification. It is written down because the difficulty of a project like this is not the idea, it is the collection discipline.
The pipeline
2 · Normalise
Different marketplaces publish on different scales and round differently. A comparison across sources is only meaningful once that is reconciled.
3 · Keep the series
Never overwrite. The point of the project is the previous value, so an update is an append.
4 · Notice the change
Flag the movements that do not look like the item's own normal behaviour, rather than the ones that cross a fixed threshold.
Constraints it would have to respect
- Where a marketplace publishes an official interface, use it, and stay inside its terms.
- Observe at a human cadence. A rating that changes over weeks does not need to be sampled every minute.
- Watch a stated list of items, rather than sweeping a catalogue.
- Store aggregate figures — the rating and the count — not the text of other people's reviews.
The part that is genuinely hard
Identity. A listing can be edited, merged, re-used for a different product, or taken over by another seller, and when that happens the series silently becomes a series about something else. A change detector that cannot tell that apart from a real movement would mostly produce false alarms.