As a seller, I detest buyers who waste my time. I tend to price my cards with the assumption that some discount will be applied. If I priced my cards accurately (at the lowest amount that I would accept), I will get incessant messages to further lower the price. By listing the price higher, I will almost always end up where I wanted.
There are ways to tactfully negotiate with sellers. It requires respect for their time, a clear offer, and an ability to follow through on your word when you make an offer. Almost always, the problem lies in one of these three areas - disrespectful messages, unclear offers that require the seller to counter against themself, and not sending an offer once it’s been agreed upon.
As a buyer, I almost never send messages. If I disagree with the list price, I send my best offer or close to it. When I do send a message, I try to be very straightforward and thankful for their consideration and time.