Then, this forum thread was helpful for getting MarkDown to work properly in MarsEdit. You have to set the default editor mode to HTML text, to stop it messing around trying to turn the post into HTML.
However, at £24 on the Mac App Store, I don’t think I’ll be buying it. It seems pretty rusty – MarkDown is becoming pretty standard for blog and note writing, but it doesn’t do any syntax highlighting, and injects HTML when you drop a picture in place or use shortcuts for formatting. The ‘Perfect Preview’ post was written 4 years ago, and claimed the developer is working to streamline the process.
I like the idea of having an offline post editor, that offers a fast, accurate preview. If I find nothing similar, I may re-evaluate.