They therefore fall under the GNU General Public License, version 2. This means you are allowed to sell the extension as long as you do not change the license terms.
Your customer is allowed to do anything with the extension, as long as it does not violate the terms of the license, including uploading it to the public TYPO3 Extension Repository.
Remember that our software is GPL software, released under the GNU General Public License, and that when you extend TYPO3 CMS, your extensions are automatically subject to this license. This does not force you to share your extension, but it should inspire you to do so, and legally you cannot prevent anyone who gets hold of your extension code from using it and further develop it.
The TYPO3 CMS Extension API is designed to make it easy to share your work and make use of the software of others. TYPO3 CMS is open source software, and a community effort. We rely on you and everyone else in the community to develop it further.