Buy orders are confusing.
You are missing the point that another seller may want their money NOW and not wait for (now 2) players to be willing to pay 10s. If they see your 5 silver sitting there for the taking (SELL INSTANTLY) they get 5s now.
If you offer a good enough price on the buy order, the impatient seller is more likely to take it rather than place a sell order and WAIT for more return.
A smart TPer always places buy and sell orders and waits for the profit.
Fate is just the weight of circumstances
That’s the way that lady luck dances
So there are two ways to buy/sell using the Trading Post. As a buyer, you can choose to buy something instantly, which using your example would cost you 10s.
But if you don’t want to spend 10s on an item, you only want to spend 5s, you can place a buy order and wait for someone else to sell it to you for that 5s.
It’s basically a waiting game, if you’re willing to wait for your item and pay less you can place a buy order. You’re risking it never being filled of course, but if it is you’ve paid less. On the other side, a seller who lists for 10s is willing to pay a fee and wait for the higher price, rather than selling to the buy orders instantly.
So again – buy orders for a seller are instant, but for a buyer you have to wait for a willing seller. Normal listings for a buyer are instant, but for a seller they have to wait for a buyer.