Are you using the old form or the new one?
The old form is dependant on Flash and Adobe recently released a new version. If you are not up-to-date, you cannot upload. It has happened to me.
Since I switched to the new listing form, Adobe Flash isn't as important to the listing flow.
Also, is Safari (assuming you use that) completely up-to-date too? I had real trouble with it for many weeks until I realized I wasn't using the most recent version.
