A marketplace listing usually has one seller occupying the default purchase button while others sit behind a link. Which seller holds it is decided continuously, and prices follow.
One listing, many sellers
Marketplaces group identical products into a single page rather than giving each seller their own, so shoppers compare one product instead of dozens of duplicates.
That design requires choosing which seller a purchase defaults to, because the page has one button and several merchants offering the same item.
The overwhelming majority of orders go to whoever holds that default position, which makes it the most valuable placement in the entire marketplace.
What the selection weighs
Price including delivery is the largest factor, but it is not the only one, and the cheapest offer does not automatically win the position.
Delivery speed, stock availability, cancellation rates, late shipment rates and customer feedback all feed in, because the marketplace is protecting its own reputation.
Fulfilment method matters too, since inventory held in the marketplace's own warehouses carries guaranteed delivery performance that the platform can rely on.
Why this produces constant price movement
Sellers use automated repricing tools that adjust their price in response to competitors, often many times a day and within seconds of a rival's change.
Each tool is reacting to the others, so prices ratchet downwards until they reach a floor and then jump when a seller runs out of stock.
The visible result is a price that changes without any promotion being announced, driven entirely by competition for a single button on the page.
When the position is empty
Sometimes no seller is selected, and the page shows only a list of offers. This usually means every available offer failed the platform's price or performance thresholds.
A price far above the recent norm triggers this, as does a listing where only sellers with poor metrics remain in stock at that moment.
It is a useful signal, because it indicates the current offers are outside the range the marketplace itself considers reasonable for the product.
What this means when comparing
The headline price is one seller's price at one instant, not the product's price, and the offers behind the link often differ substantially.
Delivery terms, seller location and return handling vary between those offers, so the cheapest listed figure may carry conditions the default seller does not.
Checking who is selling and who is dispatching, rather than only the number displayed, is the step that makes the comparison meaningful.