A price checked in the morning and again in the evening can differ on a large marketplace, sometimes repeatedly within a day. The movement comes from software competing against other software.
Repricing is automated by necessity
A seller may list thousands of products against competitors who change their prices without warning, which no team could track manually.
Repricing tools monitor competing offers and adjust prices automatically within rules the seller sets, including a floor below which they will not go.
Because most substantial sellers run such tools, the market consists largely of programmes responding to one another.
Rules produce visible patterns
Typical instructions are to undercut the lowest competing offer by a small amount, or to hold a position relative to a particular rival.
When two such rules meet, prices step downward until one reaches its floor and stops, which is why prices sometimes drift down through a day and reset abruptly.
The reset happens when a low-priced seller runs out of stock and the next offer takes over, moving the visible price up in one jump.
Costs enter through the floor
A seller's floor is built from the wholesale cost, marketplace commission, fulfilment fees, storage charges and a minimum margin.
Storage costs rise for stock that has been held a long time, which pushes floors down and produces reductions on ageing inventory.
So the underlying economics do set limits, but within those limits the movement is competitive rather than cost-driven.
Errors propagate quickly
Because the tools react to each other, a mistake in one seller's configuration can pull competitors down with it.
Pricing errors of this kind are usually corrected within hours, and marketplaces and sellers generally reserve the right to cancel orders placed at an obviously wrong price.
The same interaction can produce implausibly high prices on items with only one active seller and no competition to anchor against.
How to read the movement
A single observed price is a snapshot of an ongoing contest, so comparing prices across days gives a much better sense of the real range than any one check.
Sharp changes usually indicate a change of seller or a stock position rather than anything about the product.
Watching the range over a period is therefore more informative than reacting to a single figure, and it costs nothing but patience.