If you just want to move something on but don't want any form of payment for the item you can choose this option in Step 2 of the advert placement process, the first question on this page asks 'What kind of ad is it?' the available options include 'For Sale', 'Free to Collector', 'Swapping' & 'Wanted'.
If you are using the Friday-Ad App to place a new ad, simply slide the FREE toggle next to the price field to the right in the Place an ad screen (which appears after you click the + plus icon).
You can edit an existing ad by tapping the profile icon to view My ads. Select the ad you want to Edit and be sure to Save changes at the end of that process.