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I have asked this question on Discord to one of the most prominent members of Steam Community Market: the creator of https://github.com/JustArchiNET/ArchiSteamFarm and an administrator of https://steamcommunity.com/groups/archiasf (more than 1.5 million members).
He confirmed that the description is misleading: the implementation of buy orders is not what one would expect.
Basically:
with 30 unrestricted bots, you could set 100 buy orders at 0.01€, 0.02€, ..., 0.30€ with each bot, for a total of 4.65 €,
then perform transactions at prices lower than the bid, skipping the queues of people with higher buy orders, whenever a seller nonchalantly sets a sell order below the bid, maybe expecting the Steam Market to work as in the description.
For reference, if you try to buy an item, the dialog box contains the mention of a maximum price: you are guaranteed to spend at most the amount mentioned in your buy order.
This is the most you can end up spending as a result of this buy order.
Items will be purchased at the cheapest price available, so the order may end up costing less than this amount.
In contrast, if you try to sell an item, there is no option of a minimum price. To the contrary, it is written that the buyer will pay the exact amount mentioned in your sell order.
I believe the system is set-up this way to passively enforce https://help.steampowered.com/en/faqs/view/61F0-72B7-9A18-C70B#restrictions .
Currently we are restricting Steam Wallet balances to $2000. In other currencies, the limit will be set to a value close to $2000. We also limit the maximum price of a single listing on the market to $1800 (or similar for other currencies). You will be restricted from listing items for sale in the Community Market if the existing balance in your Steam Wallet, plus the sale price of the item(s), would together exceed the $2000 limit.
Even if this asymmetry between buy and sell orders is intended by Valve, I am surprised that pre-existing buy orders are seemingly given an inconsistent priority when matched with a sell order.
Indeed, as I mentioned in a comment below the question, I have placed hundreds of buy orders in the past, and I have always paid the exact amount mentioned in my buy borders set at (or below) the bid, never less. So my experience is consistent with Archi's statements: a sell order which is below the bid seems to be matched with a buy order:
whose price perfectly matches the sell order,
and which should not have been prioritized over the highest buy orders (at the bid).
I will try to conduct an experiment once I have access to 3 unrestricted accounts.
By monitoring the real-time view of "recent activity" for sacks of gems, we can witness an event consistent with Archi's statements, where:
the first buyer in the queue at the bid (0.30€) is called uNNamed,
there is one buy order fulfilled at 0.29€ between buy orders of uNNamed.
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