You could subscribe again to rent 25 spaces. I do not believe free players get any auction hall slots. Turbine is kind enough to let you keep five slot for being a former subscriber.
Just be glad you are not playing Swtor. You get to keep very little when you drop to Preferred status. Swtor basically has Free little better than nothing. I would call it nothing+ - Preferred (people that spent dollars) - this upgrade gives you a little more nothing++ - subscriber who rent almost everything.
Unless stated otherwise, all content in this post is My Personal Opinion.