CMS Made Simple Forums

I'm currently developing a site which needs a place where people can offer items and can search for items other people are offering, like a market.

I'm thinking about installing a forum like PunBB ore something else but am wondering if someone has an other idea on how I can create a "market" on my site

Have you made a look at catalogers module?

jus have but is it possible to have others beside the site-owner to add collections?

From frontend - no, from backend it should be possible (with some permissions).

Have you tried Upload Module that is what the theme site uses...