Everyone, including buyers, wants positive feedback. and it's wrong for sellers to demand it. As the OP put it, it depends on how the seller words his/her request for a positive feedback.
It's easy to see how it can be a headache for genuine sellers when some immature buyer casually leaves a negative feedback without proper cause. Even with the best of intentions by an honest seller, things can go wrong occasionally. So it's not unreasonable for some sellers to say something like "Before leaving a negative feedback, please give us a chance to correct things".
Before eBay disabled sellers' ability to give negative feedback to buyers, many sellers gave rataliatory negative feedback to dissatisfied buyers. Many also openly made threats on their item description page to give retaliatory feedbacks. This is a blatant disregard for all ethical business practice.
Even now, when it's no longer possible for a seller to give negative feedback to a buyer, some sellers are still openly threatening to give retaliatory negative feedback. I make it a point never to buy from such sellers.