Inside the riocar manual it says the warranty is not transferable from one user to another.

Hmm. Interesting. I wonder if that matches up with reality, though. I mean I guess they have a record of me being the owner of certain Empegs, but who knows if they're actually checking.

If they started enforcing this, it'd just be screwing the buyers anyway, not the sellers. It's the buyer's responsibility to ask if the warranty will still be valid and to ask SonicBlue/Empeg if they'll take it.