If you set a fixed price than this is what you want to pay/receive.
The market procedure will try to find a match.
There is no guarantee there is one.
for each and every offer, there must be a counter offer to make a deal.
fixed price ones are harder to execute.
Market offers and requests are matched by sorting them on a first come first serve order.
if there is more supply the price will go down and if there is more demand, the price will go up.
This is the "normal" market working, like in the real auction markets and the stock trading markets.
The complication is in taking quality into account.