I think if a user made a purchase and seller leaves positive feedback they should have the right to give as many positive feedbacks they want. Im saying this as an eBay seller. I have repeat customers and I always leave them positive feedback to show appreciation.
If you are not on DT do whatever you want.
If you are on DT, and want to remain there, then you have no such right.
This is why theymos' grand plans for bitcointalk always come unraveled...