All of BuddyPress and WordPress is under the GPL license. You can sell your themes without issue, as long as you comply with the terms of the GPL license. There are plenty of resources around the web which will help you understand.
Currently there is no facility to sell themes from this website, so you will need to find somewhere to do that, perhaps on your website.
so I even can change the default theme and sell it?
Might not make you a lot of friends though 
@ferhat – for your reference check out https://wordpress.org/extend/themes/commercial/
at the bottom, you’ll find how you can be listed there.
@jeffsayre that was a well-written post, thank you.
You must include a haiku!
