Does a ISV have the right to add a watermark to the export of users content, like video, images or pdf’s for paid noncommercial users?
The idea is to prevent commercial users creating and selling the exported content and to offer commercial export licences.
An example of this is Office 2007 Home and Student or academic versions (licences) of software packages.
A "right"? You can make your software do whatever you want... It's not illegal to watermark anything your users put through your software.
Now if users will accept it, that's a whole other question. Also up to your discretion is if you tell your users that you are watermarking exported content.
If you ask anyone "would you like a watermark or not?" they will say "don't do it!" So asking customers is no good.
Do you really believe people will rip you off? If so, does that mean there's a problem with the business model in the first place? Most people don't naturally want to rip you off. Are you afraid of accidental rip-off, and if so will a watermark prevent it?
What are you trying to gain with the watermark? If it's obvious will it ever be acceptable to your honest customers? If it is acceptable, it's probably acceptable to resell it or whatever you're afraid of, so have you solved anything? (and +1 to both answers above)