What must be free?
- the app
- submitting an ad
Assuming your ads are meant to make a profit from something, it's good business practice to pay for ads. Just like you expect your customers to pay for your stuff or services.
What is a good free classified ad submitter to post my ads?
Thanks

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic