Am I allowed to add my application to the Buster OS and offer it a an img download file for purchase?
I appreciate some people regard the Rpi as free to use but I am adding (hopefully) added value.
As mentioned that other question/answer was for a non-commercial product
I'll trace out the context here for you and point you in the right direction, but as joan implies, if you are going to do this on a serious scale you should probably consult a lawyer versed in such things.
the Buster OS
Buster is the nick (or "code") name of the current version (10) of Raspbian, sort of like how various OSX versions used to be called Big Tiger, Crazy Cat, etc. This is not an unusual practice; in theory they are code names used in development.
The reason Rasbian 10 is called Buster is because the current version (10) of the Debian OS is code name Buster, and Raspbian is really a re-packaging of the armhf compilation of Debian (armhf being the hardware platform). IMO referring to GNU/Linux distributions as "Debian OS" and "Raspbian OS" a bit of a misnomer; they are all fundamentally the same operating system and smacking "OS" on the end of the distro name is a newer practice. Of course that's superficial hair-splitting, but there is an important legal point if you wish to release your own distro based on one of those:
Raspbian is bound by the licencing restrictions of Debian which are determined by the licensing restrictions of GNU and Linux, two independent legal entities. What they actually licence are distinct pieces of software; Linux governs the linux kernel and GNU owns copyright to much of the rest of the fundamental bits such as the core system libraries.
However, what's perhaps more important about GNU is that they are the authors of the GNU GPL (General Public Licence), which is used by pretty much all the other system software and most if not quite all of the distro packaged applications. The license holders in this case are a hodgepodge collection of independent people and organizations. It actually doesn't matter much who they are though, because the terms of that license allows for re-use (under specific conditions) without applying for permission to anybody -- something which helps to explain how things got to be this way: Because the distributions from the beginning (Debian dates back to 1996) intended it to be so. They and GNU and Linux are largely copacetic on this point (although some of the players might want to interject some more fine tuned verbiage there).
It is important to note that there are actually several flavours of the GPL and you do need to read up a bit on that as I will not regurgitate it all here other than to say that the "full version" (there's also a "lesser" version) of the GPL 3 is the one you should be most concerned with because it is the most restrictive and the other flavours are compatible with it (meaning, if you meet the conditions of the GPL3 you will meet the conditions of the others as well).
some people regard the Rpi as free
Well, no, Rpi = Raspberry Pi, a brand of hardware, and so far I have not gotten any for free.
But that is actually not what is meant by "free" in this context, just as "free speech" does not refer to speeches made for no money -- ie., in the words of the founder of GNU, it does not mean "free as in beer", by which he meant, it is not something no one can charge money for (observe the double negative).
People can and do in fact charge money for distributing GPL licenced software, including a lot of Rpi vendors who sell pre-made SD cards. They don't, however, have to pay anything to the copyright holders of the software and if you want, you can copy that card and sell copies yourself without paying them or anyone else, because this is part of what the "free" licencing makes legally possible.1
"Free", "open", and "libre" are all terms that are a variation on the theme. Understanding the distinction between them is not critical to understanding the GPL, however.
As a final important note, "open" is a term that can be applied to hardware as well. Just as open software made possible the evolution of GNU/Linux distributions (without which the Pi would probably have never gotten off the ground), so too open hardware has in the past made possible things such as the personal computer revolution of the 80's, when anyone could design and sell a PC style system for which Microsoft Windows, the OS, was designed.
The important bit here is that the Raspberry Pi is not based on open hardware, and consequently some of the bits of software required to run it are not licenced that way either, namely the firmware.
Here's the licence for that stuff, which fortunately does not require permission or fees either but does have some clear restrictions.
Finally, beware that Raspbian historically has included some pretty non-free stuff such as Wolfram Mathematica. I'm not sure if this is still the case but you should keep that in mind and decide how you are going to handle that (eg., just leave it out, none of it is critical).
The topic has been discussed several times on Opensource.SE, e.g. here. In short:
If you wrote an application, you can keep it closed source, provided it's compatible with all the libraries you have used.
If you added kernel patches or drivers, you'll have to publish the source code for those.
If you sell a product with an embedded GPL software (Linux) or sell a complete bootable image, you need to provide the source code for all that GPL software you're selling. You could tell your customers you used Raspbian version so and so, but in the end if the Raspberry Pi foundation stops distributing it, you will be personally liable towards your clients if you can't provide the source code to them. So, be sure to at least keep a copy of all the sources.
And of course, remember that most information you get on the Internet for free (including answers here) is not legal advice.