Expire meta tag will tell google to delete your page from their database, and so on for every page including the expire meta tag, you are basically telling google that the document isn't valid, your SEO will probably suffer. Don't use expire meta tag!
Websites that serve free ads eventually get extremely popular, so you should think about an automated process of deleting ads older than free trial and deleting ads after paid period ends, It is better for your users to reupload their ads instead of not being indexed by search engines.
Usually websites with free ads, let users use vote links for their ads, and every 24h or 12h or whatever your cookie wants, they can vote for the ad and higher it's position of course there are stickies which have unreal votes and always on top too.
So you have 2 options for the best performance.
One : Make a script which takes care of ads older than trial period, or desired period by the client, and remove them automatically.
Two : Make a vote system and delete ads which doesn't have votes more than x time for example. So the way your ads shows would be defined by the votes they've earned, and stickies would be again always on top.
About the renewal case, you woun't really have a problem with it if you go for the first option and make a script which will delete the ad after the ad expire, and warn your users that they should renew their ad before it expire.