We won't comment on whether this is a good idea or not, but if you have the rights to all the costumes, sounds, and scripts inside the project you package, it is possible to sell it.
The open source components of Scratch and TurboWarp used by the packager are available under permissive open source licenses that allow you to use, modify, distribute, and sell your projects with almost no restrictions. There are no license fees and no royalties. It is also worthwhile to note that these licenses explicitly do not offer a warranty of any kind; the authors are not responsible for damage caused by things like bugs.
If you used costumes, sounds, or scripts created by other people, make sure you have permission from these people to sell their work. Most things you find on the Scratch website are supposed to be available under the CC BY-SA 2.0 license which technically doesn't prevent you from re-selling the work, but it requires attribution and the share-alike clause may have significant license implications on the rest of your project. Project creators can grant you additional permissions on top of the CC BY-SA 2.0 if they choose.
Note that the Scratch trademarks such as the Scratch Logo, Scratch cat, Gobo, Pico, etc. are an exception to the CC BY-SA 2.0 license, and you might not have permission to use them.
For more information, see "May I use / remix Scratch support materials, sprites, images, sounds or sample projects I’ve found on the website?" and "Can I sell my Scratch projects?" in https://scratch.mit.edu/faq
If you make money off of the packager, you are encouraged to donate to us and the projects we rely upon. ❤️
We are not lawyers. This page is not legal advice. If this is important, talk to a real lawyer.