When I'm truly interested, the conversation is basically . . .

Me: If I were to commit right now, what is your absolute best price?
The normal counter: If you commit/buy now, I would take $X
Me (If I think their price is reasonable): Would you take $X minus $Y (in a pin, I might go $100 - $200 less than what they say), then we work things out from there
Me (If I think their price is unreasonable): Well, I think you price is high based upon my research for this title sold recently in similar condition (I did my homework ahead of time). I'd be willing to offer $Y as my best price. Depending upon response we either work it out, or I walk away with no hard feelings.

Actually, I used that exact verbiage recently when helping a friend by his new pin. Seller was asking about $600 too much IMO based upon the game's current value. He came down to about $100 higher than what I told him I thought the game was worth, and deal got done. Of course, this occurred face-to-face, so neither of us took things personally or out of context at the time.