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 Panel -> Settings -> Switches in order to let your topic submitters directly collect payments without involving you. When this switch is on, two additional fields will appear on the submit topic 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 topic 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. Any HTML will work in the alternate button code field, but javascript is still forbidden for security reasons.

Important Note: This is incompatible with the "shopping cart" switch -- only one or the other can be enabled. With the shopping cart, all payments go to you instead of to the submitter (normally used when you're doing all the submitting).