Can derivitive Smartapps and DTHs originally free and covered by an Apache 2.0 license, be modified then charged for by another user?
I don’t see why not. You can charge for Apache 2.0 licensed software and and modified or changes software is considered a derivative work. Of course, you would have to include the copyright notice and disclosure. 2.0 does require that.
It depends on the exact details of the original license. Sometimes yes, sometimes no, it just depends on the exact wording.
For example, it is common for someone to include some Apache software in a package and to have the freely usable rules apply to that code only, but not to additional code that has been added. It’s why you have to look at the actual license with the original code.
It would appear without adding specific language to the license, that yes it could be modified and sold as long as the Apache 2.0 License is retained in the code.
Code which is made available under the Apache 2.0 license can be sold to another individual, regardless of modification. The code has to carry attribution.
If you want to know more about widely accepted Open Source Licenses, you should look at the OSI’s website https://opensource.org/