Read the description for the sub-forum you are trying to post in. I'm pretty sure it will tell you that you need to have a paid membership to post an item for sale in the classifieds of the various sub-forums.

As well, I'm pretty sure there is something in the forum rules about just arbitrarily posting an item for sale in the sub-forums that are not for that purpose.

A non-paying free membership doesn't get you any perks.