Even though FlightGear is released under the GNU General Public License (Version 2), it's still gotta be morally wrong to claim that it is yours, and sell it?
Flight Pro Sim (Google it!) is a "fork" of FlightGear that you have to pay for, with no reference at all to FlightGear. According to the GPL, you can redistribute the source code and binary code freely, you may modify it and rename it, and the sad part is... It never says you can't sell it. Go to the site, try to find where the source code links are. Don't you think it's wrong? (Even though it might be legal.)