When I added a 128 bitrate version of my song for use as a preview, underneath where it plays (after a customer clicks on the 'Buy this file' link) it says "you may download the preview file to play locally"

I don't want people to be able to download it to play locally! I want to sell my MP3 files while allowing people to listen to the track in its entirety before they purchase it. Is there any way to prevent this?

Also, the reason I have the sample at 128 is because when I went to make 64kbps copies of my files, I got an error message at the bottom, and it wouldn't create a 64kbps version for me. Is this where the problem lies? How do I fix these things?

unicorncove, i think this forums posting just slipped our attention and I was looking thru the older posts to see, if we missed something:

the preview files are available publicly anyhow - means you can not prevent people to download free previews! So, it is best to create short previews, like 20 second samples of your songs for that purpose!

Google for Audacity, which is a nice MP3 tool, which helps you to create such previews!

Ralf