Can I sell a theme I developed using TwentyTen
Short answer: Yes.
Longer answer: Just because something is released as open-source/under the GPL doesn't mean you can't put it behind a paywall. All WP themes have to be licensed GPL if you distribute them, because they are derived from the WordPress code, which is itself GPL, but you can still charge for it.
Your css and images, however, do not have to be released under the GPL - you could do a split license.
See this excellent article by Mark Jaquith: http://markjaquith.wordpress.com/2010/07/17/why-wordpress-themes-are-derivative-of-wordpress/
You can even sell wordpress and twenty ten unchanged while you got it for free. There is no need for you to create a theme on your own to sell it.
But bottom line is: It depends on many things, so no clear answer can be given to your question.
As far as you are concerned about a legal hint on this, you should ask that question your lawyer with your concrete theme at hand and a concept how you plan to sell and distribute your theme. Next to that you should carry a copy of wordpress and it's license term(s) with you so that it can be checked against. Normally your lawyer can aquire this from the internet directly, but in case not, just take it with you.
In general, the GPL (that's the license WordPress and it's derivates like add-ons often called themes or plugins are distributed under) allows commercial distribution and modification but there are also some terms you need to take care of in respect to the original authors. That's to protect the freedom of the software. It's not only you who is free to sell worpdress, it's everybody and the license has been written to protect this freedom ongoing. That means for the users you distribute under the GPL to, as well.