why i can not add new topic in the market place.

Onli members with a certain post counts can start their topics in the marketplace.

Please read the rules and regualtion of the market place.

If I'm not wrong you need to at least reach a "Contributing Forumite" level to be able to start a thread in Marketplace. This is to prevent spamming. Anyone correct me if I'm wrong.

yep.you will have to get at least 50posts before you can start starting a new topic in the markteplace.

also,posts in welcome page and misc section are not counted ^_^

Read it here. We have experience people who sign up just to sell stuffs and not contributing to the forum.