i take a list of orders from a few grocery stores in my town and go to the market. i've got some orders from people who'll pay £1 for 10 potatoes, some that will pay 90p and some that will pay 80p.

i meet with the farmers. they're offering 10 potatos for £1. My order s at £1 gets filled. Theyh wont sell me any more, because more people have arrived who bid £1. Still More buyers arrive, armed with orders from people prepared to bid £1.10. Those buyers get their orders filled until yet people arrive and start bidding £1.20. potatos are in demand. those orders get filled and £1.20 is the new price.

Later, all the people offering £1.20 have gone home...there's only me left. THe farmers have some potatos left, but no buyers at £1.20 and no buyers at £1, so they lower the price further . I get my 90p order filled. noone else arrives, so farmers lower the price again, and they aceept my 80p offer..

i dont know if that makes any sense ?

its gets a litte more complicated, in that the owner of the market where the deals are done, must commit to filling everyone's orders - so they have some juggling to do . they take a small margin in between the bid price and the offer price, in return.