For a classifieds-type website, you may want to let people directly sell items without having to become a middleman collecting and distributing the payments yourself. Turn on the "send purchase payments to submitter" switch at Admin -> Settings -> Switches in order to let your listing submitters directly collect payments without involving you. When this switch is on, two additional fields will appear on the submit listing page: a paypal email field and an alternate payment button code field. If they fill the paypal email field with their paypal account email, a paypal payment button will be automatically generated for them and displayed on their listing details page. If they don't have paypal, they can fill in the alternate button code to have that payment button shown on their details page.
Important Note: This is incompatible with the "shopping cart" switch -- only one or the other can be enabled.