MarkdownLib is a library which converts rich text within a Text Suite compatible application (e.g. TextEdit, OmniOutliner and others) into Markdown. This is a fairly rudimentary conversion to Markdown, but it serves my needs.
Enter the following command in the Terminal application to install the latest version of MarkdownLib on your machine:
curl https://raw.githubusercontent.com/alldritt/AppleScriptLibraries/master/MarkdownLib.applescript | osacompile -o ~/Library/Script\ Libraries/MarkdownLib.scpt
MarkdownLib provides a single public handler: richTextToMarkdown(rich text object reference).
use AppleScript version "2.4" -- Yosemite (10.10) or later
use MarkdownLib : script "MarkdownLib" version "1.0"
use scripting additions
tell application "TextEdit"
MarkdownLib's richTextToMarkdown(a reference to document 1)
end tell
See also this post and this post on the Script Debugger support forum.