can I sell mobi-files (and epubs) in my shop? legal issues?

Hi there,

This is a question concerning the legal rights and the format mobipocket itself.
We produce epubs and mobis for a customer. Now he asks me, if it is ok for him to sell the mobis in his shop.
I am not sure if there are any legal problems in doing this.
As much as I see the format mobi belongs to Amazon.

I am sure, that Amazon is not happy if some else sells mobis, but might it be a legal issue?
I have not found anything that says, that it is illegal so sell a mobi-file, but you never know...

thanks
Georg