Coin is under the GNU General Public License ("GPL"). Can I use it in my proprietary, commercially sold application?

No, the GNU GPL restricts the applications that can use Coin to have to be Free Software, and otherwise compatible with the GNU GPL.

The distribution and availability of the Coin library under the GNU GPL is primarily meant to be a service for those who write other Free Software libraries or applications. If you write proprietary software and would like to use the Coin library, see the next question.