Perhaps you want to allow your users the option to purchase things from your site with their Activity Points. This tutorial deals with that feature.
First, you'll need to enable the Purchase with Points feature and set up user group settings as you want them. Please note that once Purchase with Points is enabled, it is a sitewide setting and will allow users to purchase anything that is at a price on the site - ads, marketplace items, subscriptions, etc. This is for the default product and not having to do with third party products as those vendors would need to deal with Purchase with Points for their products. If looking to use this with third party products, please contact those vendors.