As each post has a banner space that is bought for advertising - you as the owner of the post will get an income from the impressions while you own the post.  This increases your cashable balance which will be used to exchange for BBS Tokens. It is very important to open all posts as no impression is registered if the post is not opened.

If you buy a post and someone buys it from you then you will get your money back plus a profit and that way you will increase your community balance - which will enable you to buy more posts to increase your cashable balance and/or community balance.