Classified ad

Hi, I want to add a classified ad section to my website were people can sell or swap musci gear. I would need a form for them to fill in but more importantly, their ad needs to appear alomst immediatley in a specified catagory. ie

If someone was selling a guitar, then they would fill in the form, upload an image and when pressing submit, their ad would appear in the guitar section of the classifieds.

anyone have any ideas?

thanks peeps