The above advice is great, but thought I’d point out that if you (or anyone else reading this) are thinking of selling forked themes on ThemeForest, we generally don’t allow that (I work for Envato).
While it’s totally legal if you fork it correctly, we’d only allow it if it added significant additional value over the original theme. The design would have to be totally different, to the point that it would considered a separate product that was unrecognisable from the first.
Basically, we don’t allow people to take an existing product, modify it a bit and then sell it. Basing your theme on a starter theme such as _s is fine though. Not sure what other marketplaces allow (maybe check with any you are considering selling on), but you’d be fine selling it on your own site.