How terribly difficult would it be to use Flash as a means of selling
downloadable MP3s? I can't figure out how to lead the user to paypal and then
check for a completed, successful transaction so they can then download the
files. Does anyone know of any books, tutorials or the like that could guide
me in this new adventure? I am comfortable with Flash and actionScript.

Getting flash to register a sale in PayPal is not the hard part. Flash (just
like any other page) sends the vars through https to PayPal. the hard part if
getting feedback from PayPal. Take flash out of the picture first, to get this
solved, since the problem is the same without it - its just one more thing to
think about. Next go to your PayPal management tools and check the FAQ as they
might mention something like this. You are looking to retrieve records of a
sale throough means of a var. They must cover something like this. Good luck