I don't think I would delete an answer by someone selling their software, but I might downvote it in some situations.
I think paid material can be a helpful answer, as long as it's clearly described so that people can decide whether they want to pay for it.
If someone asked for a specific tool like something that teaches stroke order using musical notes, then I would welcome anyone who is selling such a tool to post an answer. (I made that up, by the way. I have no idea if such a thing exists or would be useful.)
On the other hand, if someone asks for something more general like flashcard software, then I'd probably downvote an answer that was just a link to some paid software with no description of its advantages over the free options.
Of course, I might delete the post for other reasons. If the person posts links to their software when it's irrelevant to the question, then that smells like spam to me.