Can I sell projects made with the packager?
We are not lawyers and this is not legal advice. If this matters to you, talk to a real lawyer.
Yes. If you have the rights to everything inside the project (its costumes, sounds, scripts, and extensions), you can sell a project packaged with the RemixWarp Packager.
The open-source components of Scratch and RemixWarp that the packager bundles are released under licenses that let you use, modify, distribute, and sell packaged projects without fees or royalties. Those licenses also come with no warranty. Some parts use a "weak" copyleft license such as the Mozilla Public License 2.0: grossly simplified, if you modify those specific files, you must share that specific code under the same license. Simply packaging a project does not trigger this.
Assets and other people's work
If your project uses costumes, sounds, scripts, or extensions made by other people, make sure you have their permission to sell it. Most material on the Scratch website is meant to be available under CC BY-SA 2.0, which does not forbid resale but does require attribution and carries a share-alike clause that can have real implications. Creators can grant you extra permissions on top of that if they choose.
The Scratch trademarks (the Scratch logo, the Scratch cat, Gobo, Pico, and so on) are excluded from that license, so you may not have permission to use them commercially.
For Scratch's own guidance, see the answers to "May I use / remix Scratch support materials..." and "Can I sell my Scratch projects?" in the Scratch FAQ.