The reason you can't post is due to new member restrictions. To be considered a "full" member, you have to have been a member for 90 days AND have made 50 posts. Full members are able to create new threads in the Marketplace while new members cannot. We created this limitation to stop scammers from signing up and immediately posting content for sale in the Marketplace. By making members wait, we cut down on a lot of fly-by-night sellers.
However... you bring up a good point in that you aren't trying to sell but wanting to post a "Want to Buy" thread. Give me just a bit to look at a solution we implemented at the RPF that may work here.