As we are planning to shortly launch Windows VPS I'm trying to source some plesk licenses. As far as I understand, there are no Plesk resellers so we have to go direct.
Is their sales dept usually really slow? Maybe im just increadibly inpatient but it's been several days with no response.

Parallels definitely is not the quickest company to deal with. That said, try giving them a call - their telephone reps (account managers?) are pretty good and quick to assist.
-mike

Official Parallels hosting partners may not resell licenses to companies deemed 'outside of their facility', which I imagine is what you're referencing. If you can start downstream of someone, it will be astronomically cheaper. I'd second making some phones calls if you want to set up your own agreements, it never hurts to stay pro-active. :)

Parallels should be approached directly
Establish a relationship with a dedicated sales professional
We're in our 4th year with them