As far as I know you are free to use any song or backing track you create in BIAB in any way you choose including selling them. Here is what the FAQ (http://www.pgmusic.com/salesfaq1.htm#22) says,

22. Are songs that I create with Band-in-a-Box copyrighted?
The arrangements made by Band-in-a-Box are yours, and your songs may be used freely as long as they don't infringe upon the intellectual property rights of others.