JR wrote...
I'm not sure what rights you have to protect the application you created but,
I'm not sure it's legal for you to sell "your" application in the first
place, since you are using a copyrighted program (Microsoft Excel).
....

By that 'logic', Microsoft would own the copyrights to all books,
papers, articles, etc. composed or edited with Word.

An .XLS file (or a .XML file) is separate and distinct from the system
in which it was created and in which it would presumably need to run.

That said, I wouldn't buy any .XLS or .XLA file. Inability to create
true standalone applications implies a certain lack of professionalism.
Also, there's absolutely no sure way to protect IP distributed in the
form of .XLS, .XLA or .XML files.