Search results are, by default, ordered randomly. We felt that this was the fairest way to present our listings to members. However, when we say 'random', it is randomly set on a per session basis. This means that if you do a search, it will be randomly ordered the first time you visit the site but then fixed for the duration of your session.
If you come back on another day and do the same search, you will see a different set of results, or rather the same results, but ordered differently.
Also, there is the option for members to display the latest listings - look for a checkbox under the keyword search that members can tick to display the latest listings.